For most of the industry's life, retail had a plentiful supply of labor for critical tasks that was absolutely free. The source of that unpaid labor? Customers.
Shoppers drove the last mile to and from their house to the nearest distribution hub (in this case a store). They paced the aisles and picked the merchandise from the shelf. They walked products from shelf to point of sale and then to their car before driving off, perhaps to another store.