@Smartiepants: Women aren't as badly represented as you like to think they are, just because they don't match YOUR perfect idea of how women should be portrayed in games. There are plenty of games with great female characters, and if YOU don't know about them and YOU don't like them, that's YOUR problem.
A: Because I may not own the game the person(s) in question are playing. I'm not exactly swimming in cash here. B: With a game I do own, it might be interesting to see how someone else handles an in-game situation differently. C: I enjoy the commentary.
Also look to the numerous "why do people watch sports?" analogies already posted in the comments.