Hooligan has it about right.
Many (many!) years ago (60-70s), in Area control the "D" man was a procedural controller and Radar was just used for the climb/descents to get aircraft to/from their levels. Both controllers worked the same frequency, and the "D" man was in charge - he choose the cruising levels, and allowed Radar to do the tricky bits where procedural couldn't work (e.g. get somebody through someone else's level). Also, the radar systems did not have full coverage, and could not provide a 100% service - they were also inclined to fail, sometimes at very awkward moments, and no back-ups!
The callsign "Airways" was used to indicate "procedural" control, and "Radar" to indicate this vectoring process.
Much the same applied in Approach, as well, although there was sometimes a separate frequency for radar.
Over the years, things have evolved, Radar is (almost) everywhere (at least where there is significant traffic), and there is now the idea of a "service" type.
It is the "service-type" idea which has come in to make the picture clearer to the "man upstairs", and the ground callsign is just a (confusing?) left-over.