The importance of rest day is often overlooked in the mainstream fitness communities, the focus is go-go, no excuses, push or you're weak. While it's awesome to push yourself and work really hard, it's also equally as important to rest your body and let it recover from these bouts of intense exercise.
 When I talk about intense exercise I'm talking about lifting heavy, intense interval training, workout sessions that leave you breathing very heavily or quite sore. These sessions get your heart rate going, release a lot of hormones including cortisol, now cortisol is a stress hormone so too much of it (even if it's from a positive experience like working out) can be bad for you. This increased cortisol can result in:
-Increased weight gain
-Poor sleep
-Low energy
-Increased anxiousness  
Now these sound like the things we're trying to avoid via exercise! 
Rest days are important in so many ways they aid in the ability to rebuild muscle, when you workout you're tearing the muscle, and to make it stronger/bigger/more toned it needs time to repair these tears. Usually 24-48hours betwen working a muscle group. Rest and recovery ensures you have enough energy for your training sessions. It can also help prevent  injury from overworking muscles, or being too tight and stiff from previous training. 
Now a rest day doesn't mean sitting on your butt it can involve: gentle stretching like a yin yoga class, foam rolling and postural work, a walk, a leisurely bike ride amongst many other things.Find a strategy that works best for your rest day and helps you feel your best.