If you mean Google website login, that step is needed because the email address is used to determine which identity provider to use. E.g. I have three different accounts that branch off from that same initial login flow.

One is my person "gmail.com" account, and the other two go through enteprise identity providers related to my employment and their G-Suite licenses. So after I put in one of these three email addresses, I get prompted for the appropriate next step. Only one of them involves giving a password to a Google server. The other two are redirects to completely separate login systems operated by my employer.