A blog is for authoritative content from small list of privileged authors (the authors are more important than the commenters; there is an author/reader separation). Blogs often have a single or few authors.
Blogs are best suited to provide announcements, news, and expert thoughts or ideas.
Why use a blog?
Why use a blog as opposed to a forum or a wiki? talk about experience, hear news or [expository?] information from authoritative authors
In a forum, you are asking questions or discussing subjects. In a wiki, you are recording information in articles for people to use.