A while ago Steven Dillard of Southern Appeal gave a talk to a local chapter of the Federalist Society about blogs. It was an interesting talk (or so I assume from the transcript), and it's good reading whether or not you know a lot about blogs. In answering why he blogs, he said the same thing I tell people:
I don't want this to sound snotty, but there aren't many people in Macon [or Los Angeles] who are interested in hearing my thoughts on the original meaning of the Ninth Amendment or discussing whether the Privileges and Immunities Clause should replace the Due Process Clause as the jurisprudential basis for incorporating the Bill of Rights to the States. The blogosphere, however, brings me into contact (on a daily basis) with those who are interested in discussing and debating such esoteric issues.
I've developed a lot of "virtual" friendships with people, and I'd say that's the best part of blogging. And a blog is a great screener. If someone likes my blog posts, chances are he'll like me. Heck, I'm actually better in person. (Sometimes I feel like I come off like a robot on the blog.) Likewise, if someone dislikes my blog posts, the person probably won't like me. Anyhow, do check out the notes from his talk.