The Downside of Critiquing
It helps you identify weak points in your own writing. You know that whole plank/speck thing? All those things you can't see in your own writing are easier to see in someone else's. And the cool thing is, the more you do it, the more likely you are to catch them in your own work.
It helps you learn from people's strengths. Like, I'm terrible with the descriptions. So when I'm critiquing for someone whose good at them, I'm all, "Oo, how did she do that!" And because I'm in critiquing mode (instead of reading mode) I actually pay attention to the answer.
It helps you make friends. People like it when you do something for them, and they almost always offer to pay it back. It's an easy way to build solid relationships, which for an introvert like me is critical.
But the downside to critiquing is this: