1. From the common sense, this is to let people with ulterior motives know, don't play tricks, there are people at home.
Culturally speaking, that is to say, put a pair of shoes at the door, because shoes are worn on your feet. You all know that feet have beriberi, but we don't call it beriberi here. It's bad luck and the air is dirty. Because unclean things are so-called ghosts and demons. They like to go to popular places, and the bad luck in their shoes will give them the illusion that there is no popularity here. So putting shoes at the door is actually an exorcism.