Why should I "rest" meat after cooking?
There are a couple of reasons why 'resting' meat is a good idea. First, as the meat cooks the muscle fibres contract, which forces the juices out. Letting the meat rest helps the muscle fibres relax so the juices are re-incorporated into the meat and not lost on the chopping board, which would happen if carved immediately.

The second reason to let meat rest is to provide an evenness. Basically as the meat rests it continues to cook slightly and it allows the meat to gain an overall 'doneness'