But if they don’t know they have to knock “shave and a haircut” first, your job gets a lot easier and you’re dealing with a lot fewer nuisance password promptings.

Very good explanation. And the benefits are even greater, because there is absolutely no response until the entire secret knock is correctly used, the random guy trying to get in doesn’t even know if there’s anyone at that address. (In fact, set up correctly, they won’t even know if there’s really a door there or not)