Food continues to cook for some time after being removed from the heat because it is already at a high temperature.  At this stage and in roast meats especially, the food juices are agitated by the heat.

If you carve the meat at that very moment, most of the juices will end up on the cutting surface and the food will be dry.

This is why you must allow meat (especially large cuts) to ‘rest’ for 5 to 20 minutes - depending on size - before carving.