Steve Yegge has a very nice writeup on why one should blog and I will not repeat here what he says there - it is a must-read for all people who think blogging is a completely wasteful activity. I would just add that I personally use my blogs and website as a sort of a Pensieve (what Dumbledore uses to store some of his memories in the Harry Potter series of books), except that I like to record my thoughts too instead of just memories. Sometimes I look at what I wrote a few years ago to see what I was intrigued by or interested in back then and occasionally I can't help but be embarrassed or amused by what I see. It still does not stop me from blogging though, mostly for the reasons that Steve points out in his article.
I have found out that blogs tell me a lot more about a person beyond what I can learn from everyday interactions with him or her. For example, Vikgup, Arpana, Anupama and Mridula have blogs that showed me aspects of their personality and areas of their interest that I had absolutely no inkling about from my conversations with them. If you do not blog yet, go forth and blog (and send me the URL for your blog).
By the way, despite my (admittedly mild) pleas to the contrary, Mark has added this blog to Planet Classpath as well. If you see posts in Planet Classpath via this blog about stuff not related to GCJ or GNU Classpath or Free Software or even technology, you now know who to blame for it.