Well, it's a question I get asked all the time in my job. In fact, we're currently reviewing whether we think a company in our position needs to create "official" blogs at all (more of that later... *taps nose*)
For me, it's a case of getting on my virtual soapbox. My friends and colleagues *will* be pleased.
Most of my work here is (obviously) commercially confidential, and there is no way I can talk about it. But there are key issues that crop up in the context of doing my job on a day-to-day basis that are worth chatting about. And as my job is about stakeholder engagement, using digital media, how post modern and ironic that I have a blog about it.