Have not monetized a blog yet personally... but I've worked on company blogs for others. All the reasons stated in the post above apply to one blogger or another.
I started my personal blog just because I enjoy writing and sharing opinions. It's an extension of having been in political discussions in forums waaay more than time shouldve permitted over the years. Long as I'm at it I suppose monetizing 'em for a little profit is appropriate. My wife's grown partial to having a roof overhead, so if I spend the time it should at least pay me back.
My idea of a blog was to share my own views on topics I liked. It can be intimidating if you don't feel you know a lot about that particular topic, though. Some commenters can be pretty rough.
Blogging is a virtual representation of your business. I blog because I want to promote my company just like every one else There are different reasons for everyone for blogging. Blogging is important in many ways.