The idea seems to be that you only have to do this once, then you remain authenticated for every service that integrates with this authentication service. You

The idea seems to be that you only have to do this once, then you remain authenticated for every service that integrates with this authentication service. You don't have to remember another password or place trust in a new service not to mishandle it. This seems like a solution that completely replaces the need for every website to manage their own user signups, which is something you still need to do even if you integrate with Google/Facebook/etc. because not everyone has an account with those and signing up for one is less trivial than having a sign-in link sent to your email.