The reasons for starting and publishing a blog vary widely. In fact, I suspect there are as many reasons as there are bloggers. Some bloggers publish online newsletters for their local societies. Others write about their favorite software product(s). Still others document their own genealogy research efforts. Still others document the ancestry of famous people or post obituaries of local residents. Perhaps a few decide to write daily news or events within the genealogy community. Whatever the reasons, more than 1,700 genealogy blogs are now online, according to the Genealogy Blog Finder at http://blogfinder.genealogue.com/ and I suspect that isn't all of them. Instead, that's only the genealogy blogs found to date. Undoubtedly, there are more.
Of course, genealogy is only one of the reasons for writing a blog. The suggestions I will offer apply equally to almost any blog topic: genealogy, history, your rock band, the local Cub Scout troop, politics, a high school reunion, and any other topics you can think of.