Machining the rotors and drums is integral for providing smooth, long lasting brake service. Most car companies prefer the use of On-Car brake lathes because it provides a more precision cut for longer life. Even when installing new rotors, it is recommended that you machine the rotors to match the hub and rotor together.
Conventional bench lathes are still ideal in instances of customer drop offs, machining of drums, and "old school" technicians that are stuck in their ways.
Bench lathes are the conventional method of machining rotors and drums. Though not the recognized method by most OEM's... this is often the tech's favorite (for the old school guys).