Ever read a blog? Given that you are reading a writing magazine, the answer is probably yes. Wikipedia defines a blog as a discussion website with informal diary-style text entries called posts. For an author, a blog can be an effective addition to a publicity portfolio since they are about comment and are mostly, but not exclusively, about sharing your thoughts and activities with people. They are rather like a personal magazine, and can help develop an audience, centred on you and your books.
Each individual blog will have its own readership. What if you wanted to try to talk to groups of readers from different blogs? Even if you only touched on literary ones, you would never be able to contact even a small minority of the readers.