There are two main reasons to let your pork tenderloin rest before slicing. Letting the tenderloin rest at least 5 minutes allows the carryover heat to finish cooking the tenderloin to the target temperature AND it allows the juices inside the meat to redistribute and be absorbed by the meat fibers, resulting in a juicier slice of meat. The USDA recommends at least a 3-minute rest time, but I almost always let my pork tenderloin rest a full 10 minutes.