While exercise is important for gaining muscle and losing fat, it does place physiological stress on the body. After you exercise, the muscles' fuel resources are depleted and minor damage has occurred. This suggests that your muscles need to be repaired and replenished.

Of course, the damage and depletion allow your muscles to adapt, as long as you simply allow your muscles to be during this state for a brief period of your time.

Exercise tears down your old, less adapted muscles that the body can build more functional muscles. This process is commonly spoken of as remodeling said, Paul Anthony Bennett. However, unless you provide your muscles with the proper materials, they'll not be able to remodel themselves.