Vanilla in chocolate cake, why?

No, you don't have to use vanilla, but it helps when you're baking a cake or brownies or cookies, instances when that 3/4 cup of cocoa or chocolate will be competing with flour and eggs and butter and sugar to be noticed by your taste buds. Not only does vanilla boost the flavor of the chocolate, it gently coerces the other ingredients to play well with each other.

When the chocolate has little or no competition for your taste buds, as with something like ganache, I leave out the vanilla.

You could make both with and without and see if you find a worthwhile difference. I would think it would contribute a subtle depth of flavor. Just like adding coffee to a chocolate cake.

If someone put vanilla in their chocolate cake recipe they did for depth and flavor. Someone was thinking in complex terms rather than the mundane.

Vanilla is a great enhancer of flavor, just like salt (good comparison, TiggyBee). It's great on its own, of course, but in chocolate cake, it's meant more to bring out the other flavors. Some chocolate purists will tell you that vanilla isn't needed if your chocolate is high-quality. I happen to agree with this. If you're using very high-quality chocolate in your cake, you probably won't miss the vanilla. Instead, you'll be enjoying the whole gamut of flavors that come out of good chocolate. (Think fruity, nutty, or earthy notes.)

Per Harold McGee: "Added with a light touch, vanilla can contribute a sense of depth, warmness, roundness, and persistence to almost any food."

Great question indeed. I can't find an obvious answer. Cook's Illustrated explains about every ingredient choice for their chocolate cake recipes, except for the vanilla (which is included in each recipe!) I agree with TiggyBee's impression of why it's in there, it accentuate other flavors. You could bake a chocolate cake w/ vanilla and w/o and see if you can detect a difference!

Wow, great question! For me, I think vanilla is a little like salt, not adding it when called for, there's just an indescribable something missing. It seems to be a great balancing flavor. That's the best way I can word it!