When you place your meat down on a hot surface like a skillet or in a hot environment like a smoker, the fibers of the meat tighten and the moisture inside the meat begins to be forced out. Without resting, all of that moisture is lost!
Brining works by the interaction of salt and liquid. As the salt gets inside the muscle fibers of the meat, it holds on to liquid. During the cooking process, your meat will always lose moisture and brining is a great way to maintain it.