Q. Why do you let roast beef "rest" after cooking and before carving?
Being in the oven is no fun. The high heat, in addition to browning the outside of the meat (and thereby generating most of the flavor) also pushes the juices in the meat this way and that. When the meat comes out of the oven, it continues to cook as the heat in the outer portions of the meat continues to work its way towards the cooler center of the roast.
At the same time, as the heat evens out in the meat (and as it begins to cool), the pressure in the meat diminishes and the fibers are able to hold more water. So the juices in the meat distribute themselves more evenly as the roast rests, providing you with more or less evenly juicy bites, instead of some bites moist and some bites dry.
By the same token, once the roast has rested, you want to use a very sharp knife to carve it. A dull knife will exert a lot more pressure and will squeeze those lovely redistributed juices out of the roast just as if you had squeezed a sponge.