Transponder on 7700 allows all ATCO's working the area to see the aircraft instantly on whatever radar they are working. Remember that ATCO's that are not directly involved but working the airspace above or to the sides of the area will be able to ensure traffic is routed around or away from any direction the mayday aircraft is going or may need to go. It instantly flags up the problem aircraft on the screen even if they weren't aware of the problem straight away.
Or so I have been told....