As it states on the OAuth website:
OAuth is a simple way to publish and interact with protected data.
One way to use GitHub OAuth in a non-GitHub website or application is as an identity mechanism. You give your application’s users the ability to use GitHub to authenticate who they are. This saves you from having to deal with passwords and all the mess that goes along with them. It also gives you an avatar for them as well, so you don’t have to store and police potentially abusive avatar images. If you’ve seen a “Sign in with GitHub” button on a website such as https://travis-ci.org or https://exercism.io, then that’s what they’re using GitHub OAuth for. Additionally, we have information on how to do this in our developer documentation.
But how to use OAuth depends on what your website or application does. Does your website or application need to see its user’s GitHub data? Then GitHub OAuth might be the right solution. Does your website or application need to calculate spaceship transfer orbits between Venus and Mars? Then GitHub OAuth is less likely to be the right solution.
I hope that helps!