For now really the only advantage I can see is that users, if allowed, can have multiple different blogs for their different blog niches. Whenever I get around to finishing the next x.X.x release there will be more features for content discoverability like similar blog posts to the one that is being read and more.
I haven't used a profile post in a very long time so they could also allow less characters than blog posts, but haven't checked into that personally.
That would be due to the blog description field having a hard limit of 255 characters. I'll probably have that changed in the future for it to be an option in the acp to set it to whatever an admin wants, but for now due to the addons default design on the front end and where the description currently appears it will stay the same for some time. Probably until after XenForo 3.0 when the new default style is released to cut down on the amount of styling changes that would need to take place.