They're right, Chris. The "rest" is important for any larger cuts of meat, especially. When the meat is cooking in the smoker, the juices (rendered fat and connective tissue) are pulled from the meat, and rise toward the surface. When you foil wrap and keep it insulated, it actually stops the flow of those juices toward the surface, and allow them to be reabsorbed by the meat. It will continue to "cook" due to the trapped heat, but as this stops, you'll have very moist meat due to the juices being reabsorbed by the muscle meat. This also helps tenderize the meat.