User authentication and identity should be a part of the platform.
As a website/app developer, I need a way to associate multiple user sessions to the same account but I often do not care how they authenticate (as long as it is secure). Currently, I have to register with one or more identity providers (google, facebook, github, etc.) and maybe implement login+password sign in as well. This is cumbersome and is a waste. There is also a chance that the user does not have a facebook or google account, etc.
As a user, I want to simply sign in into any website or application with one or zero clicks. I want to use my preferred authentication method independent of what the developer has implemented. I don’t want to remember whether I signed up with username+password, with google, or facebook. Sometimes I want to use different identities for different websites, though (e.g., personal vs work identity).