I can't speak for the millions of other people with blogs out there, but I started my blog in an attempt to stop littering my friends' email boxes with 10 emails a day about new music, albums, tour dates, etc. I figured it was easier to have one place they could come if (and when) they wanted to rather than bombard them with email...
When I started getting free promo CDs in the mail from bands, that pretty much sealed the deal for me. I don't think my opinions are more important than anyone else's, but if you're gonna send me free music in return for posting my thoughts now and again, why would I want to stop?