As foods cook on the grill, they lose moisture and become dry. This process begins on the surface of the food (which is closest to the heat) and gradually progresses toward the center. Moisture either evaporates or is driven toward the center of the food. So when any food is removed from the grill, it is less juicy on the surface than it is at the core. Letting the food rest before cutting allows moisture to redistribute from the center back toward the surface. If you cut into steak immediately after grilling, the moisture will be unevenly distributed: The surfaces will be drier than the center. Immediate cutting also drains excess juices at the center because the saturated muscle tissues cannot hold the extra juices that were driven there.