I blog for the same reason the rest of the world (who isn't a techie and doesn't have much to say):

1. I want attention from people; to be known and remembered.
2. There's a lot I want to say, but rarely know how to put it, whether people will read it, and how to make it interesting, so i usually give up halfway through and it turns out crap.

More specific to me (me, me, me!):

3. I like to learn, and assume other people do.
4. I love to write, though it tends to be the hard to get started and organise my thoughts in a linear fashion in the first place.
5. I'm a student of human nature, people's reactions, and manipulating such things. I would love to be able to phrase such in a blogging atmosphere, but I haven't stumbled upon the way to do it whereby people read it :-)