Why do you need a round trip ticket to rent a car with a debit card?

Car rental agencies require you to show proof of a round trip ticket if you want to pay with a debit card. I don't have a problem with this, I just wonder why,|||Because if you've already paid money to return home, you're more likely to return the car.|||Maybe to know that you are returning to the particular airport for a return flight. Otherwise you could just get in the car and never return.