Question: Do I really need to let meat rest before slicing and serving it?

Answer: You should. When meat is hot, its proteins are tight, and all the juices get pushed to the middle. Letting the meat rest for a few minutes allows the proteins to relax (which makes it more tender) and evenly distributes the juices. Resting time depends on meat size: A whole turkey might need 20 minutes, while a steak that serves two will only need 8 to 10 minutes. If you're worried about it getting cold during that time, keep it warm by loosely tenting it with foil.

Source: Diana Sturgis, Test Kitchen Director, Rachelraymag.com