Golf is a very precise game which requires the execution of a series of coordinated movements. These movements can be difficult to understand, visualize and even harder to "feel". Nothing verbal or visual can replace having the feel for a correct swing. This is why drills are so important. They allow the student to feel the correct move.
When a teacher explains with words alone, it seldom results in the action the teacher and student are trying to accomplish. There is without a doubt that the use of video is extremely beneficial to accelerate the learning process. Video allows the student and teacher to isolate a particular area of the swing. The beauty of a drill is that it is specific to a small part of the swing.
When working on a swing change, a student will make the fastest progress when focusing on one specific goal. Focusing on too many changes at once on the practice tee will make progress slow and uncertain.
The key to making drills beneficial is to do them on a regular basis. Don't just use drills alone, incorporate them into your overall practice. Once the sensations are ingrained, leave the drill, coming back to it only if the old, bad habits return.