There are different use cases.  Some people like consuming information through video or audio media.  That's why most people watch four hours of TV per day.

One of the reasons mobile is so widely used is that you can take it with you.  So if you are on your commute, for instance, you might play a video (such as the one referenced in this post) instead of trying to read an article on a tiny little screen.

So there are good reasons other than "efficiency" for there beng an audience for video blogging.