Thanks to these interface, the user and role systems can be completely decoupled from the implementation. This is what enables AsgardCMS to give you the option to either use Sentry or Sentinel out of the box.
You are absolutely free to implement other user/role/authentication providers. You could for instance make an implementation for the popular packages Confide and Entrust, or maybe you're satisfied with the basic laravel implementation.
To make a new implementation its very easy, you simply need to implement the said interfaces and place the implementation in the Repositories/ folder.
For your inspiration you can check how Sentry and Sentinel are implemented: