Over at bookends, the agent there posted a question, asking: why do we blog? She answered (in short) that for her it was to do with networking, publicity, popularity, things that will help build her career.
I'd have to say that my own answer would be very similar. Originally, I set the blog up in preparation of having my own author web page. Obviously I haven't actually done anything about it yet (they're not cheap, and I'm not earning anything from this career yet!) I knew that it was becoming expected of authors to have blogs these days. Much easier to blog than answer fan letters and emails I suppose.
Secondly to that, I did want to have somewhere a prospective agent could find out a little bit more about me in a more relaxed environment than what is presented in query letters and proposals.
Thirdly, (and this is where the enjoyment comes into it) my husband, friends and family can really only take so much of me talking about my writing. And half the time they don't know what I'm up to with it anyway. Its not because they don't care, or aren't supportive, its just because they don't find it anywhere near as damn interesting as I do. I'm sure once I actually get published, they won't be able to hear enough about it. In the meantime, I really think they think that I'm not actually ever going to get published, and one day I'll just give up and go "oh well, that didn't work out!" So if I want to go on and on and on about sending out letters to agents and how excited I am about my WIP, I can. And there's nothing anyone can do about it!