Pure and simple: it keeps the dog from doing the WRONG thing!! It’s a placement command as well, allowing you to put the dog EXACTLY at the spot to move sheep in the desired direction.
Ideally, the dog should be in a “sit” or “down” as opposed to a “stand/wait.” Why? Those little paws in an excited dog will take baby/puppy steps and not truly stop. We are not looking for a pause but an absolute ceasing for movement.
We recommend you take the dog to a place with distraction and work on it there: OR, if ball/frisbee is the thing, train it while playing.