[redsolver]

It does prove that you own the specific matrix id you are logging in with to the website. It can for example be used as an alternative everywhere where "Sign in with Google" is used, so I don't understand why it should be useless for applications.

[ushakov]

"Sign in with Google" also grants you access token you can use to access Google APIs on behalf of the user

sorry, but your concept doesn't

[MakiXx]

Aren't you supposed to use "Sign in with Google" or which ever service to prove to the app that you own your unique id (e.g. email)? If you want Google APIs, then that's a completely different scope here.