The use case I’ve generally seen that leads to this design is supporting SSO from different providers.
So, you ask for email first, then you can look up the user and see if the use a password to login, or if they need to be redirected to some other system to manager authentication.
This is correct. It's called "identifier first", and it's a feature I've seen organizations request pretty often when they have to support multiple providers and auth schemes.
I worked at a company that supported authentication with any of the InCommon Consortium members, and the solution was to provide a drop-down[1] where the user could select their institution (mostly academic or academic-adjacent) and then be directed to the provider to authenticate. As you might imagine, this only worked when there were a handful of institutions and the users were somewhat sophisticated. As of now there are nearly 600 institutions. With that many selection in the dropdown and less sophisticated users, that UX was unsustainable.
So, you ask for email first, then you can look up the user and see if the use a password to login, or if they need to be redirected to some other system to manager authentication.