It’s often useful to have multiple accounts, and using +addressing lets you do that without creating multiple email
Addresses.
User+whatever@gmail.com will get delivered to user@gmail.com. User@gmail.com1 won’t let them create an account.
Oh, but SSO was the point of this thread, so it’s not clear that my solution will help here. They’d still need to make every email address unique, one way or another.