RUDAS
Procedures are generally used only when vectors are not available (except for training purposes obviously).
Vectors are generally not available either because of equipment failure, staff shortages or just because there is no radar available at the field in question.
I think DME arcs are mainly used when you need to reposition the aircraft laterally where a straight in is obstructed (whether by terrain or other procedures) and also where the levels available over the airfield for aircraft to position outbound are limited.
An example would be the DME arc at Biggin. Aircraft cannot be positioned straight in because of LCY (& LHR?) traffic, and manouevring over the BIG to go outbound would interfere both with SIDs and missed approaches, because of the base of controlled airspace.
Timothy