To preface, I am not an author. In fact my skill with the pen was much more proficient when I was a youngling, but alas I have returned to write.
There are many reasons, although I don't believe there even needs to be one. If you want to blog, blog!
To improve my writing skills
I'd love to get back into writing. The ability to convey, and pass information to others is a valuable skill that I very much wish to posses. If I'm able to help another understand, learn, or just discover something, I see this as a massive benefit.
Gain a better understanding of the blogosphere
What goes on behind the scenes for blogs? How do people network? How can you tell you're doing a good job? I could easily find these through research (and likely another person's blog), however I want to learn the answer to these, and more first hand.
Learn SEO, and keyword research
I want to help others get on, and use the web with a good chance to find, or have likeminded people find them. SEO, and keywords are a large factor in this, as a good portion of the web is now found through search engines.
Get an alternate income stream
The least likely of the bunch, but if I could pocket even a little from this blog, I'd be a happy chap.