Companies will often want to limit the email addresses that are used for Account Users.
Use Case 1
The company may want to prevent inviting/creating users tied to consumer email addresses like @gmail.com or @yahoo.com.
Use Case 2
Both the company and business accounts may want to ensure that account users' email addresses explicitly match defined email domains for that account. For example, the account Acme, Inc. maybe want all account users to have the email domain @acme.com (or subdomains like @na.acme.com). In this use case, the company's account managers can still manually add non-matching domain users. This is useful for granting outside contractors access to an account.