I would assume it's for the benefit of the aircraft following or crossing beneath, most probably on the same frequency.
Exactly! Not only is there a greater separation requirement, but the term is also used as situational awareness by ALL aircraft on the same frequency.
As a pilot of a light, single engine GA aircraft who often flies into Boston Logan airport, I am very appreciative of hearing the suffix, "heavy," included in those monsters' call signs. A wake turbulence encounter with a heavy's wing-tip vortices would really be a bad day for any small aircraft, unless of course, the pilot were a very skilled aerobatic pilot.