Support for managed user authentication by unique ID other than email address
Email address is not a reliable identifier in our world because it tends to change for the end user at certain times:
User decides to legally change their name (via marriage or other situation)
User goes on a leave of absence (in which case the email address is removed or temporarily disabled in their HR system)
When these changes happen (usually initiated from their HR system), it will percolate downstream to update their other applications. Their IdP manages user identities by a unique identifier called GUID along with the user's email address.
Because of the unreliability of email address they prefer to authenticate into their applications by the GUID instead.