This doesn't use Github pages isn't it? The title kind of misled me.
See https://github.com/codemy/writefully#why-dont-you-just-use-s... for more differences from GitHub Pages.
With writefully the idea is you get real business objects you can build business intelligence into. So for example I only want users who buy a piece of content to be able to view it, I can do that with associations or some sort of logic to detect who bought what. It's difficult to do that sort of thing with static sites.