The point is that it allows you to remember which provider you used to sign in with and you’re not having to guess whether you logged in with Google or GitHub or whatever else.
You go from delegating some of the most important elements of your life to a third party who can take your account away at any moment with no recourse (an extremely vulnerable position), to having complete control over all those accounts, with zero extra effort.
It's very confusing.
When I used third party login the whole point was that if I saw it was available, I was sure I'd use it, so I didn't need to store or remember anything. If I tried and it didn't work, I knew I didn't have an account.
So to see these responses was very odd to me.