1. You have to get noticed to get promoted.
2. You have to get noticed to get hired.
3. It really impresses people when you say "Oh, I've written about that, just google for XXX and I'm on the top page" or "Oh, just google my name."
4. No matter how great you are, your career depends on communicating. The way to get better at anything, including communication, is by practicing. Blogging is good practice.
5. Bloggers are better-informed than non-bloggers. Knowing more is a career advantage.
6. Knowing more also means you're more likely to hear about interesting jobs coming open.
7. Networking is good for your career. Blogging is a good way to meet people.
8. If you're an engineer, blogging puts you in intimate contact with a worse-is-better 80/20 success story. Understanding this mode of technology adoption can only help you.
9. If you're in marketing, you'll need to understand how its rules are changing as a result of the current whirlwind, which nobody does, but bloggers are at least somewhat less baffled.