The purpose of creating a drifting throttle was to use saturated steam. If you used the main throttle, you'd get superheated steam. No need for a separate drifting throttle on a saturated engine.
Another use for it is to supply a small amount of steam when drifting downgrade. In small amounts, saturated steam carries oil better than superheated steam. When running a locomotive down grade, the proper way to handle it is with the throttle slightly open to lubricate the valves and pistons and to prevent the pistons from drawing exhaust gasses into the cylinders.