In the UK self-checkouts in big stores (such as LIDL or ASDA) are very popular. Many of them are card-only. What is the benefit of providing such checkouts over cash-or-card checkouts? Paying with a card doesn't seem to be faster, unless it's a contactless.
There are a couple of advantages that I can think of.
Since the machines are less complicated because they don't have to handle cash, they are less expensive and require less maintenance. Machines that handle cash require lots of moving parts.
Cash machines require lots of employee interaction. The machines need to be stocked with cash each day, and at the end of the day the cash needs to be taken out and counted. With a cashless machine, the computer does all the work.