I feel like this has become such a big thing because of SSO, very few apps don’t offer SSO and thus they all do this. Truth be told tho, I’ve seen apps that have both fields and as soon as it detects a SSO domain it hides the password field, so it could go either way…
You know, I have been annoyed by this trend but I had not considered this. I think you are absolutely correct. It’s to support redirecting them to an SSO provider based on their email address.