Darren Strange, UK Office Product Manager, wrote an interesting post about why he blogs.
I started blogging as a complementary aspect to the job that I do which is supporting Microsoft development technologies. I wasn't "tasked" with it by my management and indeed if I ever was then I would probably stop as that would defeat the spirit and point of blogging for me. It is entirely voluntary. Having said that it is recognized by my managers as something worthwhile, which is helpful.
I know from working with colleagues and with my customers (mostly on a one to one level) that I often know things that they didn't and that they find useful or that help them solve a problem. Blogging gives a channel to offer that knowledge more widely and if even 1% of the people that read a post find it even vaguely useful then that has achieved something.
I would like to blog more often and with more comprehensive technical posts.
Why don't I?
Time. You can know a topic quite well or have a piece of information filed away in your inbox somewhere but when you actually want to shape that into something ready for the wide world of the Internet it can take quite a bit of time to knock into shape.
At a personal level, it is as simple as the satisfaction of having provided something that helps people to achieve things. Job done.