Training hard for 3 to 4 weeks and then resting or backing off on the volume has been around forever. Legendary bodybuilding trainer Vince Gironda supported it. As did the Eastern Bloc trainers. It simply allows you to attack your goals fresh.
Resting will release the accumulated fatigue throughout your whole body. Hard training is a cumulative stress on your whole body.
Not only the muscles, but also the joints, ligaments, bones and probably most importantly organs and glands get fatigued. Once they are rested your progress will be better.