If you don’t have reliable refrigeration, you can’t keep milk fresh, so if you want to have milk on hand, you have to get new milk daily. If everyone in your neighborhood needs milk daily, it makes perfect sense to have a guy deliver it instead of everyone making a separate trip every day just to get milk. It also makes sense that the guy who shows up literally every day would be the one fucking the troops’ wives. It’s all pretty intuitive once you think about it.