Resting chicken allows the juices to redistribute throughout the meat. When meat is cooked, the juices are forced to the center of the cut. If you cut into the meat immediately after cooking, the juices will flow out and the meat will be dry. Allowing the meat to rest allows the juices to be absorbed back into the meat, resulting in a juicier, more tender final product. It's especially important to rest chicken because it tends to be leaner and can dry out easily.