Grocery shopping requires customers to adhere to a social contract, such as leaving behind used tissues or wrappers that you've eaten out of, which can lead to germs being spread over the cart. This behavior is offensive to employees who use the cart after you, and disrupts their workday. If the store offers disinfecting wipes and hand sanitizer at the entrance, it's good manners to use them. Not every store enjoys the mostly-cart-free parking lot situation like Aldi, but return the buggy from where it came, which improves safety of the parking lot and makes employees' shifts less frustrating.