Your federated login service!
There are many reasons why and scenarios where AuthService.net might be the answer e.g.:
Simply put it's your user database in the cloud. You register your app, get a client id and secret. When you have those details you can start regestring users, loggin in, reset passwords and stuff in an OAuth-fashioned way. You can also store small portions of data for the users. This might be a good idea if you have simple need and don't want to setup any database at all.
A perfect solution when you're building an app and need to offer a secure login!