In a domain environment, your credential is never transmitted, so HTTPS does not improve authentication. In a domain environment, mutual authentication is built into the Kerberos protocol, so HTTPS does not provide that. In a domain environment, HTTPS encrypts the transmission. However, WS-MAN already applies encryption to a level of the transmission.
So in a domain environment, HTTP gets you everything you need in terms of protecting authentication; HTTP does not encrypt the entire channel in the same way, but whether or not you feel you need that is very much subject to your circumstances.
HTTPS is used when a domain is not available. It provides protection for Basic authentication, where credentials would otherwise be transmitted in the clear, and it provides mutual authentication. This is pretty clearly explained in “Secrets of PowerShell Remoting,” which is free (Resources menu on this site).