When the meat is cooking, the juices move around inside of the meat. Allowing the meat to rest allows all of the super flavorful juices to find their home within the meat. If you immediately cut into a piece of cooked meat, all of the juicy deliciousness will leak out of the meat and you'll be left with a dryer and less flavorful meat. Most meat only needs a few minutes to rest before cutting into it. Usually, about 5 minutes will do the trick. If you have a very large cut of meat, increase the resting time to 10 or 20 minutes.