If you don't want to use electricity, you can have a knock. What reason would you want to be notified of unknown visitors knocking on your doors when you're aw

If you don't want to use electricity, you can have a knock. What reason would you want to be notified of unknown visitors knocking on your doors when you're away? The only people I can see fitting this description are delivery drivers & salesmen? Further, what if you're home and your phone is in another room, is on silent, or is dead? What is you have a spouse or housemate? Do both of you get notified? And there's also a subset of people who get frustrated by having to use QR codes in restaurants, who I suspect would be equally frustrated by this.