D/F may be not so necessary at TMA airfields where most of the traffic is in the tram lines but almost essential at airfields with a lot of GA activity and in Cat G airspace.
Been saved many a time when when the bearing of an unexpected inbound indicated that it was in the climb out with one about to roll! As others have commented, useful when aircraft reporting downwind are actually more like base leg; location of aircraft on the ground etc etc.
Should be mandatory at ATC Units in uncontrolled airspace for my money!