How is a two-step identifier better than one in one when you have a password manager?

I use a browser-based password manager. For most websites, the login process is as follows:

  1. Go to the login page. The username and password fields are visible.
  2. The identification information is automatically filled in by the password manager. If more than one identifying information is associated with this site, the first set is selected.
  3. (Optional) Select another set of identification information from a drop-down list.

However, some websites (for example, Google Accounts) have a two-step identifier. First, fill in your username and click next, a new page will load, then complete your password and press next, you are now logged in. (Note: I am not talking about two-factor authentication.)

With a password manager, this becomes:

  1. Go to the login page. Only the user name field is visible.
  2. The user name identification information is automatically populated by the password manager. If more than one identifying information is associated with this site, the user name of the first set is selected.
  3. (Optional) Select another set of identification information from a drop-down list. The user name of this set is populated.
  4. Click Next. The password field is now visible.
  5. The password identification information is automatically filled in by the password manager. If more than one identifying information is associated with this site, the password of the first set is selected.
  6. (Optional) Select another set of identification information from a drop-down list. The password for this set is populated.

Therefore, if there is more than one identifying information associated with a website, it is more difficult to connect with a password manager. If I do not want to use my default credentials, I need to remember steps 3 and 6, otherwise an incorrect username / password pair is selected.

Password managers are generally considered a best practice nowadays, including for ordinary users. So I guess UX designers have to deliberately interrupt the workflow for "a user with multiple credentials stored in a password manager". Why? What is the benefit of a two-step process that makes it preferable to a one-step process?