once an aircraft has reported 'visual' (ie can navigate by land or water to the airfield) you can assign an altitude visual. This allows ATC to assign a level that may be below radar terrain chart, however responsibility for terrain clearance is put on the pilot.
visual levels are perfectly legal (at least they are in Australia) they are used to give aircraft the most descent they can possibly give and provide the best possible service (in flight conditions withstanding).
It should also be noted that you are required to report visual in order to be cleared on a visual approach. if you are given decent visual (and not a visual approach) it would be because the controller is unable to give away the visual approach (think traffic below you!)