GitHub pages is free and highly scalable, but unless you're a paying user GitHub has some limits. So for things like very popular projects you could simply move your stuff to a server+nginx or s3+cloudfront. However, these cost money so it's hard for many to justify doing that when GitHub Pages is free. In comes Netlify which provides the "better" GitHub Pages experience with the ability to use whatever you want that can be installed using go, python, ruby, or node and the resulting markup is distributed via a CDN all for free.
With such a nice deployment story the question becomes: why not just put every website like this? Mainly because it's all based on using Git to edit markdown files which usually non-technical people have issues with. Netlify offers NetlifyCMS as a solution to this and Forestry offers a more robust version of this with a more powerful and clean editor.
If you read [The First Round Review](http://firstround.com/review), you've seen it in action.