In Canterbury Tales, Why does the goodman want the door barred in "Get Up and Bar the Door"?

This isn't from Canterbury tales. It's really an old Scottish ballad. Basically the couple is arguing about who will lock the door at night. They reason that whoever speaks first will have to lock the door. Two thieves (not sure if they are thieves but they are bad) walk into the house, eat their pudding and threaten the man's wife. Finally the man speaks up and tells them to get out. The wife accuses him of speaking first so he loses the wager and has to lock the door. The song sounded pretty silly when I heard it as a kid and it sounds just as stupid today!