Sit at the door accomplishes several things at once. It promotes calm behavior. Whatever activity we do starts out calm. We never exit a door way wild and crazy. It reinforces self control. By instilling the habit of sitting to come in or go out, a dog looks to their person first instead of bolting out the door. And most importantly, it claims space. By owning the doorway with your body, making sure that the dog stays seated, and then calling them through, it teaches them how to wait to be invited into your space. There’s no need to compete to get outside first or to get back into the house first. If you own all space, that means you get to decide who comes through a doorway and when.