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.
I get that part, but by the time you've set up this reminder, why not go the extra millimeter and actually add a password?
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.
I suppose there could be any number of reasons. Maybe they were already using SSO prior to using a password manager and it would take more effort than an extra millimeter to switch.
If you are already using a password manager, why not login using password? No 3rd party dependency and risk, and no need to remember how you signed in.
I'm not responding to the question being asked, I'm responding to the people answering it.
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.
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.