I really enjoyed reading this blog post linked from Twitter (Steve Wheeler) on why teachers should blog. I also liked many of the comments that were posted as a result, including that Blogging is fun... I used to keep a diary as a teenager, and this is a sort of professional version of that I suppose.
Importantly for me, it's a place where I can reflect on good and bad teaching experiences, write down nuggets of information that I think I may want to review at some point in the future, and it is also somewhere I can look back over in the future to see how far I've travelled as a teacher.
I related to Audrey's comments about lack of blog comments and feedback - something I also need to work on, but at the moment I'm happy just keeping a blog for my own purposes... if at some point someone wants to read it, then that would be great :)