Some more reasons:

1. The positive environment. No matter how regrettable a posting (I've done a few) there's usually someone to come along and agree, or at least strategically ignore.
2. Communicating with people all over the world. Being part of a world community makes me feel less provincial.
3. The great escape into music when I need a break from whatever I should be doing.
4. All the information. Ask and ye shall usually find. Basic facts as well as fascinating anecdotes and of course the trivia.
5. The laughs out loud. (The banjo player in the whorehouse, leave her behind unprotected, and "Now I'm a Feces" come to mind).
6. The wonderful personal stories. Even though we usually don't comment on other people's postings, for whatever reasons, this sharing of bits of our lives is touching and appreciated.