There might be more than one way to authenticate yourself to a service, especially at their scale.
For example, Google lets you use your phone instead of a password, so that means that they'll want to show that screen instead of a password prompt. Microsoft do too, and also provide options for using physical hardware keys like smart cards, fingerprint sensors, etc as the first factor authentication.
If you're using a managed corporate or education account, you might log in using SSO (single sign-on), which means you'll use one account to log in to all your services --- in this case they'll want to redirect you to that portal after you enter an email address, although often it is Google / Microsoft that provide the single sign-on account.
I'm sure there are also some other special cases which cause these screens to be different as well, prehaps when people are locked out of their accounts, or custom interactions designed for specific devices or companies.
From a security perspective, it doesn't really change too much. It does show what account names are available (e.g. if you enter in a bad email address into the Google login form it won't take you to the next step, so rather than a more vauge "invalid email or password" message you're getting "invalid email"), but you get this information already by trying to sign up with an email address that's already in use on the sign up form, so there's really no way around this on a public system like theirs anyway.
@example.comand it'd work the same way. Also, arguably, trying to prevent username enumeration with vague error messages results in loss in UX for a questionable gain in security (especially if you have a public registration page!).