By the time you make it to check-out, you’ve already given the grocery store management “free labor” by walking through the store and pulling your own items. Piggly-Wiggly was the first grocery store to push that task off onto their customers, in the 1930’s. Before that, you’d go to the counter and present the clerk with a list of what you wanted, like Sam Drucker’s store in the old show Petticoat Junction. :)
Self-service grocery stores like Piggly-Wiggly could trim costs over the old model, and by passing those savings on to their customers, became the new standard business model.