> What you're essentially saying is that "every gallon of milk is sold to a person". This is not useful information.

Only because you're ignoring the other option, that the gallon of milk DIDN'T sell. It tells you that there was demand for the milk you had to sell. If there was no customer, the milk would have spoiled. There are still people who want to drink milk. There are still people who believe you're not selling poison, that milk is a worthy thing to buy from you. That is useful information.