A blog is for [disseminating information] to an involved audience. Blogs are best suited to provide announcements, news, experience accounts or expert opinions. 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.
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.