I use my blog for a couple reasons related to my "day job." My blog talks about generational marketing and communications. I would say I kind of use it as a self-promotion tool. The job I have is a marketing director for an organization that is very focused on this topic. My blog doesn't refer to my organization, because it's not about that, it's about my personal thoughts.
I also blog because I'm a writer and I'm obsessed with this subject (generational marketing) and blogging gives me an opportunity to express these things with 100% control over them.
I started out my career 15 years ago as a journalist. I found at that time that it was very frustrating that you were always dependent on someone else to get published or get on the air. The net has brought back my love of writing and broadcasting, too, because it brought the control back to me.