I ran across a great blog titled: Post 300 or why all developers should be blogging. The reasons given in the articles are great, and I think they apply to all technical people.
You will learn by blogging, and you will keep stuff in a place that you can go back and review it, in addition to giving you something to impress interviewers. There is one more big reason, however.
You need to communicate.
I don’t think I’ve ever seen a position that didn’t require some type of communication skills. You might not need to get up in front of a group and express things (or you might), but almost everyone needs to document things or communicate through an electronic written medium. It could be email, IM, texting, some forum system, or anything else. However you need to communicate.
The more you build these skills, the better off you will be. There are definitely some very talented people that can get away with poor social skills and poor written skills, but that’s not most of us. Most of us need to communicate effectively.
To me, blogging is like typing. A developer that hunts and pecks with two fingers is not likely a good developer. And certainly they’re not an efficient developer.
The same thing comes from communicating. You need to practice it to get better at it. You don’t need to write as much as me, or write as long a posts as I do, but by writing regularly, you’ll learn to better express yourself.