Writing is generally supposed to be a solitary occupation. We sit alone in a basement room somewhere, pounding out pages and pages of brilliant story.
I don't know about anyone else, but I can't imagine writing being solitary. I need critiquers. I can generate a million ideas a day, but I can't always tell which ones are good. I need someone to read my plot outlines and point out that my character is overlooking something that should make her fighting mad, or that my latest idea about the world my story lives in is ridiculous and unbelievable. The critical input I get is necessary and useful.
But more than that, the positive input I get is invaluable. Finding out which parts others like helps guide my next attempt and helps define themes for the story. Nothing makes me excited about my story like having someone else be excited about it too.