A "blog", short for web log, is essentially an online diary. The software on which these forums is based, vBulletin, has had this functionality for many years. I was even a member of an extremely-busy site called PhillyBlog which began life with the stated intention of being blog-oriented with a secondary messageboard functionality. Ultimately, everybody preferred the messageboard (forum) format and very few really used the "blog" feature. Nevertheless, the software blog capability proved to be a significant draw and the site sported about 25,000 active, participating users, much if not most who posted regularly.
Even so, it's still an interesting and possibly useful functionality to have. Among other potential uses I can see is a content creator posting the progress of their project in the blog section rather than cluttering up threads with possibly minor updates.