Dieppe made the Normandy landings possible by showing what not to do.

First, let me deal with an unvoiced objection. Yes, the US had been doing landings in the Pacific for a good while before Dieppe. However, none of them would closely resemble attacking northern France. Most amphibious assaults in the Pacific were on much more rural areas, without built up cities or ports as options. The landings of US troops in Africa were unopposed or only against light opposition, giving little information on the subject. So, how to handle such obstacles and opportunities was largely unknown.

Enter Dieppe. Dieppe showed that even if you wanted to attack in Normandy, you shouldn't attack a port directly. The advantages to the defense (houses making makeshift bunkers, city streets funneling troops, bridges and roads being difficult obstacles to take) are too great, despite the very real need for the things a port offers (rapid movement of supplies, access to a road network, calm water). So, the Allies didn't land in Cherbourg. Also, they created the incredible Mulberry portable harbors to help handle the logistics (regardless of the efficacy of those inventions). PLUTO--Pipeline Under The Ocean--was also planned to help with the petroleum requirements.

Why do you need so much gas? Because you're going to need tanks in that first wave. You'll need them to take on enemy bunkers, to give your troops mobile cover, and to give your infantry some direct fire support—things found to be lacking in the Dieppe raid. On the subject of fire support, you're going to need something that was missing at Dieppe--effective communication between the landing forces and the naval ships offshore. Without that communication link, the navy ships would be too concerned about friendly fire to be able to provide effective support. You're also going to need air cover for a number of reasons. While keeping enemy planes away is a real requirement, the ability to interdict enemy reinforcements is critical as well. This is especially true when it comes to enemy armor, as your infantry will not have enough organic antitank weaponry to long resist an armored attack. Another handy way to keep enemy armor away is to keep them guessing about your attack. If you can make them think that this is just a raid when it's the main assault, they won't want to overcommit to repulsing it. Same goes for making the enemy think that you're going to have your main landing somewhere else and that this is just a diversion. Having elite units involved that train for special obstacles was also seen as highly worthwhile, given the successes of British commandos.

It is a difficult thing to say to any family that their child’s sacrifice was “worth it.” In the case of the Dieppe raid, the Canadian (and British, and a few American) sacrifice enabled future successes because the toughest lessons—the ones that only can be learned by doing—were learned before the Normandy invasion. Had those lessons not been so indelibly etched into the minds of Allied commanders, one can only shudder at how much worse the cost could have been for Overlord.