Beyond the the fact that it’s very important to your physiology and phycology of your human existence. Your muscles repair and your joints get to rest. When you are working out no matter what fitness genre you always have rest between sets, reps, or movements. Think of the big picture like macro, meso, and micro scales. Same with resting. You have rest periods during your workout session and no pan out to the macro scale and should have a rest day!
Resting is also important for optimal performance. And if you ain’t training to be better at somehting you need to recheck your DNA. If you want to get better resting is a must! Do things like get a massage, yoga, stretching, Pilates, or swim. Don’t forget to get some sun! Don’t over think this. Just give yourself a day to relax, unwind, and let your body and brain relax!
Remember just because you’re doing a lot doesn’t mean your getting anything done! And less sometimes means more!