This is a good site to point people to when trying to explain why anyone should start a blog:
A blog is an easy way to get started writing on the web. Your voice is important: it deserves its own site. The more people add their unique perspectives to the web, the more valuable it becomes.
Remember that your blog is yours! Your posts can contain exactly what you want them to: a sentence of text or a novel-length essay; photos and videos; audio; links and bookmarks. What you want to share is entirely up to you.