People are writing and using blogs for many reasons. Here are some links I found on Mark Wagner's blog: 10 reasons why blogging is good for you. Blogging for business: Top 10 Reasons for Blogging Why do we blog? Weblogs can be your backup brain! Blogger Moms talk about their reasons to start blogging. Why blog? Why blog?
There are also several sites dedicated to explaining why using blogging in education is a good idea. I especially recommend Weblogg-ed (I have Will Richardson's book and attended a conference presentation on blogging by him--he is one of the gurus on blogging, wikis, and podcasting). For those wanting a pretty exhaustive list of ways to use blogs in their classroom and/or teaching practices, I would suggest Anne Davis's list: Ways to Use Weblogs in Education
Blogs in Education - This page is designed to provide you some resources if you want to get started using blogs for yourself or with your students. There are articles about blogging as well as sample educational blogs. Matrix of some uses of blogs in education - This brief discussion includes a link to a graphic matrix.