This might be a somewhat strange one, but... how many open mouths you need to draw in a comic because of all the talking. Or, how important it is to be able to draw open mouths convincingly--not something that comes up as often when just doing illustrations. That was one thing that never occurred to me as being important when writing a comic until I actually got a few dozen pages into my own.
As far as learning real world things, it's nothing specific, but I feel like references to historical figures/events in fictional comics has made me more likely to go investigate those things on my own. So, I suppose the comics are indirectly teaching me about a lot of history?