Well before we touch anything else let's just step back for a second and think about this logically. Do you honestly believe that these two offensives would have been undertaken if there was not good reason to do so? That "they" would just do something for shit and giggles but had no real purpose? This isn't a cartoon, they aren't evil villains sending thousands to death for laughs. That's not meant to poke fun at you but rather the people who purport these myths; they come out for various different reasons but ultimately these two myths in particular are fundamentally centered around painting the British Generals Staff as this caricature of stupidity and evil. That alone should discount this narrative.

So now we have to ask ourselves two questions for both offensives:

1. Was the cost of not performing them valid enough to justify the potential expenditure in human life?

2. Did they do what they were intended to do?

So let's go chronologically. Was the cost of not performing Gallipoli valid enough to justify attempting it?"

I would argue certainly yes. The application of Gallipoli was what was problematic but the motivation behind it was centered in very real thinking; if anything the British Generals Staff should be praised for their flexibility. Honestly, if I can just get a momentary rant in, this confounds me to no end. Pop culture likes to paint "the Generals", notably the British, as being incompetent swines who couldnt think of anything other than to send men into the grinder; yet in the major example of them attempting something completely different, a seaborne invasion of the Ottoman Empire, they are bashed endlessly for trying something new. It's unfair to them.

Anyways, back on track, yes the motivations behind Gallipoli were sound. The Western Front, in 1915, was a stalemate and it would only be in 1916 that things would pick up again with Verdun/the Somme in the West and Brusilov on the East. The British were very astute in noticing this fact. They did not have a massive conscript army yet (that would be late 1916/early 1917) and their volunteer army was still very, very untrained. The lines in France were not moving very much and, again, they did not have a very large army. However what they did have was the largest and most powerful navy in the world. Where do they use this then? Germany or the Mediterranean. They can't do Germany because the entire principal was to not engage the German fleet; they did not want to give the Germans a chance. It was almost like a mexican standoff between their two navies and the Generals Staff, Churchill in particular, felt it was too risky. So the Mediterranean would be chosen and they would do it against, you guessed it, the Ottomans. The Ottomans with a skeleton force of a navy and an army which was nothing really to speak about; the "sick man of Europe" was prime pickings.

So what was the plan of Gallipoli? Firstly it was to relieve pressure. 1915 Russia was, in all practical senses, encircled from the Caucuses bordering the Middle East all the way up through the Balkans, Poland and Prussia, up through the Baltics being attacked, simultaneously, from three hostile powers. By rapidly attacking the Ottoman mainland, near their capital moreover, the hope was to give Russia some breathing room. More hopefully though was to swing Bulgaria onto the Allied side. Bulgaria was, at this point, a wildcard directly bordering Serbia to the Northeast and Austria-Hungary to the South. By rapidly seizing Istanbul and basically causing the Ottoman war effort to begin to waver hard Bulgaria would get in on the action. With Istanbul seized, naval superiority, the Bulgarians swarming into Ottoman Europe along with them riding to the aid of the Serbians (who were brutalizing the Austro-Hungarians 1 on 1) the allies of Germany would falter. By knocking out the Ottomans along with the three pronged (hopefully four pronged; the even bigger expectation was that with Bulgaria going a domino effect would go into place and Romania would join in too) against A-H Germany would be isolated. In other words the war in France was to be won by neutralizing her allies and making Germany collapse from all sides. That is not, by any accord, faulty reasoning.

So now we have to ask, Did it do what it was intended to do?

No. Gallipoli was, at least from what they expected, a failure. Even though Istanbul was evacuated naval invasions were just not quite there yet. Communication from the ships to the men on the ground was clunky at best and the technology was not just there yet. These same concerns would be brought up again in planning for the Normandy Invasions as the lack of decisiveness allowed was a concern; luckily for that war though amphibious tanks, modern airplanes, wireless radio, paratroopers, and a slew of other advancements made the difference. Anyways back to 1915 Gallipoli was not a total failure in retrospect though; you ask Sir Edmund Allenby and W.T. Massey, the latter who wrote a fantastic (though very...for its time we can say in the terms of racial propaganda) analysis of the Mideastern Campaign, and they would say it was Gallipoli which saved Egypt in 1915. Those Ottoman forces would not have been in Anatolia but taking part in the assault on Egypt without Gallipoli and without that reduced force (which the British handily defeated) the Ottomans may have won, or lost causing much more damage, disallowing the British to have such wild success in that theater later on.

I would echo what /u/elos_ said about Gallipoli, and if you can deal with a cut-and-paste from a previous answer, elaborate a bit on the naval aspects of that campaign and what went wrong. From this previous answer:

But, speaking specifically to the Dardanelles, the lesson of that campaign is less that Churchill was an incompetent as it is a cautionary tale about mission creep. The goal of the initial Dardanelles expedition was to use British naval power -- specifically, older battleships that could not stand in the line of battle -- to blast a way through the Straits and bring their guns to bear on Constantinople, hopefully toppling the Ottoman regime, or at least forcing them to terms. (This is not as farfetched as it sounds, because one ship, SMS Goeben, had essentially tipped the country into the war.)

The Straits were known to be mined, and also to be defended by heavy guns. In the brutal logic of the war, Churchill figured that even losing several older battleships, with their crews, was less bloody in the long term than infantry attacks on the Western front. The problem with the naval expedition was that the Ottomans had a surprisingly stout interlocking defense, both of lines of mines and also of large guns. There's also a pretty significant current in the Strait. So an initial effort by battleships to force their way through was marked by an early defeat (and a bit of timidity on the British part when they discovered the mines). The minesweepers (converted fishing trawlers) the British then bought to the Straits were deeper in draft than the anchored mines, which tended to depress morale among the crews of the sweepers. Nevertheless, they successfully swept at least two of the lines of mines before fire from shore batteries knocked out several of the minesweepers. At that point, they rethought their approach and brought in the HMS Queen Elizabeth to try to knock out the shore batteries by firing over the peninsula (from the Aegean side). This was actually fairly successful, and the Turkish batteries were running low on shot and becoming desperate before the battle of March 18, in which an undetected minefield sank three of the Franco-British battleships and forced a withdrawal. (It's arguable that had the fleets pressed on, the Ottomans might have withdrawn from the peninsula; we will never know. Ottoman morale was wavering and the batteries were nearly silenced; the fleet's withdrawal allowed them to restock and replenish.)

In any case, the failure of the ships themselves to be able to knock out the guns led to a call for a landing at the Dardanelles, using troops to secure the batteries. At that point the Strait could be swept of mines at the fleet's leisure. Unfortunately, that led to Anzac Cove and the bloody fight to follow, where far more lives were lost than those on three obsolete battleships.

tl;dr: The Dardanelles campaign was poorly planned and insufficiently supported, but it was not in its conception or aims necessarily wrongheaded. It was like many aspects of WWI, in that the technology and tactics of the time had simply fallen behind defensive technology, and in which a great loss of life would have to be accepted for even the most basic of gains.

Sup! Good to see my previous post got mentioned, and /u/elos_ is already at it!

My opinion is that both the Somme AND Gallipoli had valid strategic aims; however, the Somme was the more successful one.

Gallipoli

Having spoiled surprise with the attack on the forts, one would think that an amphibious landing in the same area might not be the best idea; BUT, it was worth a shot to take the forts out, but the second attempt (the landings) was fumbled in the attempt.

However, while there were valid strategic reasons for the operation, i.e. supplying Russia, opening the Black Sea, knocking out the Turks, I feel there were major flaws in these aims that need to be addressed.

First and foremost, while supplying the Russians was a valid aim, considering that the Western Allies needed the supplies they had JUST to equip their own forces in the West, what exactly were they going to be able to supply? Considering the corruption and byzantine politics of the Tsarist Bureaucracy, how much of that was ACTUALLY going to be supplied? Or would it wind up like the supplies flown over 'The Hump' in WWII i.e. sold on the black market? How about the chaotic nature of the Russian railways; what if those supplies simply never arrive?

The landings might take the peninsula, but then there's the rest of Turkish Thrace; also note the that Asiatic side of the straits is ENTIRELY IN TURKISH HANDS. Were the Turks just supposed to surrender, give up, with their homeland under threat, and more importantly, the CAPITAL OF THEIR EMPIRE SINCE 1453? Constantinople has the Theodosian and Inner (Constantine?) Walls, plus the massive urban sprawl within the city. What if the Turks (and the inhabitants!) choose to fight? What then?

Furthermore, the Turks STILL hold the Asiatic side; they still have coastal guns and minefields, as well as their Black Sea Fleet, which had previously pulled a Port Arthur on the Russian fleet there. Where do such difficulties factor in? The Turks still have the c. 95% OF THEIR EMPIRE ON THE OTHER SIDE OF THE STRAITS.

How about the Central Powers? Won't the Germans and Austro-Hungarians support their Ally? Didn't the Germans have military advisors there, like Pasha Liman von Sanders and Colmar Freiherr von der Goltz? What about the U-boats of the German and Austro-Hungarian navies? How about Bulgaria? They stand to gain more by aiding the Central Powers than by staying neutral; they won't help the Allies, not with the Serbs on their side! What if Bulgaria joins, as it did in September 1915? What was the plan should the Allies find themselves bottled up in Turkish Thrace, facing elements of the Armies of 4 Countries, TWO of them Great Powers?

In short, I can think of no other way to describe Gallipoli except for a Blind Alley; some good strategic aims, but ignoring huge problems. Coupled with the loss of surprise after the attack on the forts, the plan should have received MORE than a second look. They could have helped the Russians more by committing those forces and resources to the Western Front, to give greater strength to the offensive launched there. Maybe Vimy Ridge could have fallen in 1915? Maybe Neuve Chapelle, Loos or Aubers Ridge would have been more successful. German divisions could have been prevented from moving east, and more gains could have been made. Instead, they were diverted.

Can't they help the Russians MORE by attacking in the West? The Russians were to suffer c. 2.1 Million casualties that year, and NOT because of the Turks. Greater concentration of forces in the West would have forced the Germans to divert forces there, and their successes of 1915 might have been nowhere near as wildly successful as they were.

/u/DuxBelisarius discussed The Somme (and Verdun) a few days ago. His post may answer the non-Gallipoli half of your question.

http://www.reddit.com/r/AskHistorians/comments/32t8bn/world_war_i_question_largescale_offensives_like/