Two reasons. First, it will help the meat retain its juices. During cooking, bundles of muscle cells in the meat contract, forcing out liquid from the spaces between them. As the meat cools, those cell bundles relax, reabsorbing the liquid.
Second, resting evens out the temperature and the doneness. A rib roast is cooked when a meat thermometer registers about 120°F in the center of the roast—but the outside is much hotter. By letting the meat stand, you allow the outside and inside to come to equilibrium, according to Lynn Knipe, an associate professor of food and animal science at Ohio State University. The center temperature will rise by about 10 degrees after the roast is removed from the oven.
For any kind of roast—meat or poultry—Marc Forgione, corporate executive chef of New York’s BLT Steak, recommends letting it rest for at least 15 to 20 minutes. If you cooked it on a rack in a pan, just take the whole thing out of the oven and let it stand. If the roast was cooked in a pan, take it out and put it in another dish or on a board. Forgione says to put the resting roast in a warm but not hot place, such as on top of the oven. Some people put a tent of foil over the resting meat to keep it warm, but this can trap steam and render crispy crust or skin soggy. If that happens, heat up the pan drippings and baste the roast with the sizzling juices, says Forgione. That should restore the crust.