The purpose of authorizing the end-customer domain is for the end-customer users to be able to log in and access the platform with their work (Active Azure Directory - AAD) credentials. ► How to log in with Work email (AAD Account)?
Usually, customer domains are authorized, when a Seller has requested to create a platform (this information is filled in in the provided form):
If the end-customer domain is not authorized (new end-customer) to the platform, the customer will get the "Please enter valid email" message while logging in to the platform.
In more detail read the article What to do if I get the "Invalid Domain" message?