Anil Dash’s explanation resonates with me:
the reason I have a weblog, and the reason I advocate to others that they ought to maintain weblogs themselves, is because I think it’s important for people to have a place to express their opinions and thoughts, and to get feedback on those ideas. The interchange I’ve had over the past three and a half years with my readers, and with the authors of other sites I enjoy, has had profound effects on my personal and professional life. The biggest single moment that has occurred to everyone I know who actively maintains a weblog is the first time a stranger contacts you or leaves a comment where they indicate that something you wrote about touched them, or discussed a topic that they didn’t think anyone else had an interest in, or informed them about a subject that they didn’t even know they were interested in.
In short, it’s making connections between people in the same way that we make links between pages on the web.