Yan over at skwpspace.com, who also works at CohesiveFT, wrote an excellent post on 5 reasons you should blog. I would add another to his 5: I view this site as well as my blog as a central index of my activities in blogosphere and social Internet, which I want to be linkable to me.
You probably visit tons of blogs or sites, either through your RSS reader or when researching a topic. You might leave a comment here and there. In the end, you will end up with dispersed comments on multiple sites and only search engines will know about the link between you and those posts. If you want this link to be easier to see and follow, a personal blog is a great tool to help you achieve that.