Why Rest Meat?
Always rest meat before serving, this allows the meat juices to be reabsorbed back into the meat.
If you cut into the meat and the juices (and incidentally the flavour) flood out onto the plate then you’re probably not resting the meat for long enough.
When the meat is being cooked, the meat juices flow away from the heat towards the center of the meat, by resting meat the flavoursome juices are redistributed as the protein molecules relax. Some studies show that almost twice as much liquid is lost if you don’t rest the meat.
Resting Meat – How Long?
As a rule of thumb you can allow 5-10 minutes resting time for steaks (depending on size and thickness), for a roasting joint you will often need between 15 and 20 minutes resting time.
If you’re cooking steak, Gordon Ramsey recomends ‘flashing’ it through a hot oven before serving. For roasts, my experience is that the meat doesn’t cool quickly and the addition of gravy warms the meat to the perfect temperature.
More Info on Resting Meat
I found a massive report all about this from the food lab of Serious Eats – take a look here if you have the time.