Comment by jonfk

My understanding is that this is usually done by companies that need to implement delegated authorization which is the bread and butter of OAuth 2.0. By starting and implementing their authentication flows with OAuth2, you can support both delegated authorization and native authentication where your own apps are simply OAuth clients with some special casing. That isn’t absolutely necessary since you could build authentication separately from OAuth but then you would still need those OAuth2 flows for delegated authz eventually if you do.