During my mother's childhood in her country of origin, a neighbor might swing by your house and deliver a plate full of chow, delicacies or baked goods. Local custom was that when you returned the plate to them a few days later, you never gave it back empty, but loaded it with the fruits of your own kitchen labors.
When I was a waiter my boss used to dress me down if he caught me traveling empty-handed between the ground floor and the storeroom in the basement. He had grown up in a four-storey Brooklyn brownstone, he explained, and his mother would admonish him if he traveled between floors without carrying anything. "There's always something that needs to be brought up or down," she'd say.
These lessons are universal, and no one knows them better than logistics coordinators for shipping companies. If a container crosses the Pacific loaded with Toyotas and goes back empty, that's a huge waste of fuel. But despite their best efforts, it happens all the time. And even if they weigh different amounts, 1,000 empty containers take up the same amount of space as 1,000 full containers, meaning the ships are forced to make the same amount of trips each way.