I think if you're looking at their overall strategic impact it certainly hastened the end of the war, but were they "necessary", probably not. By 1944 Germany was already in a two front war and was being relentlessly bombed. Germany was hopelessly outmatched by the Soviet Union and Germany had no real chance for victory. Now that isn't to say the Normandy landings were useless, in fact they were the exact opposite, but more on that in a sec.

Stalin had been pushing for a second front since 1941. When the western allies failed to invade in 1942 as Stalin had originally hoped, he began to accuse the western allies of cowardice and that they were forcing the Soviet Union to do all the heavy lifting. Part of the reason behind the North African and Italian landings was to both satisfy Stalin and divide Germany's forces.

Now as I mentioned before Germany in 1944, when the landings happened, had essentially no chance of winning, but that doesn't mean the D-Day landings were useless. They ended the war much quicker by drawing Germany into a true two front war (I'll explain why its problematic to characterize Italy as a true second front in a second), something Germany had tried desperately to avoid.

While the late Western Front (1944-1945) tends to get overlooked, there were a number of important actions that helped crush Germany's military strength. The Falaise pocket, the battle of the Bulge, etc. All these actions drained Germany's strength and made hastened their surrender. The Western Front occupied over 1 million German troops that I'm sure the Nazis would have loved to use on the Eastern Front.

Now the reason its somewhat problematic to characterize Italy as a "true second front" is that Italy's terrain lends itself really well to defense. The Germans also had a brilliant general in Albert Kesselring who was able to take advantage of the terrain to make the allied advance up Italy slow, and costly. Because of this the Germans never paid a huge amount of attention to Italy and considered it a sideshow. Kesselring's army numbered only about 10 divisions, and while it would be reinforced and given more units, the number was never very high.