Milk left out in the open will turn sour in less than a day. Even if it hasn’t been out there for long enough to turn, animals or birds might attempt to get through the foil cap on the bottle to reach the rich cream on the milk’s surface - in 1921 there was quite a famous issue with this, as birds like starlings or tits were clever enough to figure milk theft out. If they could punch through the foil with their sharp little beaks, they could gorge on as much cream as their stomachs would hold.

A milk door provides somewhere secure, cool and sheltered to leave the milk until the homeowner can collect it. Less risk of the milk turning, less risk of unwelcome theft.

Since milk deliveries also commonly happened very early in the day, the milk door also allowed a household to return empty bottles or place their orders for the next delivery without necessarily having to get up early enough to catch and speak to the milkman. Look at this example of a milk door, and you’ll see what I mean. Can you read the writing?

A housewife (we’ll assume for the time being that it was the mistress of the household that was organising provisioning!) could use an arrangement like this to communicate with her milkman, returning the glass bottles to be refilled and telling him both what items she needed the next time he came and how much of each was necessary.

Now look at the dial again. You’ll notice that milk isn’t the only item the milkman can supply. It also has options for soft cheese, butter and eggs. You asked why there were no equivalent egg-men…that’s why. The milkman does that too.