Why Separate Email from Password

I can't talk for other places, but at the company I worked previously, it was done only because most of our users were from companies that had their own IdP (identity provider) and they wanted to login using that as SSO. Usually as a condition for the company to become our customer. Whether that was a hard requirement, checkbox ticking, or a bluff, I couldn't say because that was already set in stone long before the time the task actually reached us devs.

So if a user entered their email, and clicked next, most of the time they had to be redirected to their IdP instead of asking them for a password that we were unable to validate. Us asking for a password (instead of redirecting after asking for email) was the exception.

The most common case would be because they bookmarked something internally, and then they visited their bookmark without having cookies or some way for us to know how we should authenticate them.

For some sites it is nessesary. How else do you want to be redirected to the SSO of the company?