Scans against this port are commonly used to test if a machine runs SGI Irix (as SGI is the only system that typically has this enabled). This service is almost never used in practice.
RFC1078 -TCPMUX acts much like Sun's $/Exploits/Ports/111$portmapper$ or Microsoft's $/Exploits/Ports/135$end-point mapper$ in that it allows services to run on abitrary ports. In the case of TCPMUX, however, after the "lookup" phase, all further communication continues to run over that port.