The authentication profile function allows for SMTP auth on inbound traffic.
Example : external service provider wants to sent and receive email to your users
Possible Implementation : create Exchange or O365 mailbox and enable basic authentication as well as POP3/IMAP4 for mailbox access
create AD user to be used on Ironport for access validation
External service provider : uses AD user account and password to login into Ironport, Ironport will ask AD for access verfication via LDAP
sends outgoing email to Ironport for processing
receives incoming via POP3 or IMAP4 form Exchange or O365
I hope that helps to understand the SMTP auth use case.