In addition to better firewall traversal. Without wrapping the TURN payload in TLS, An eavesdropper would be able to see the IP address of the peer (the destination of the relayed traffic). Depending on your threat model this may or may not be important.
Excerpt from the RFC
The primary protocol content of the messages is the IP address of the peer. If it is important to prevent an eavesdropper on a TURN connection from learning this, TURN can be run over TLS.
