Short Answer: People spend more with their credit card.
History has shown that people have a less-harder time parting with their money when they spend via plastic. It’s purely a psychological thing that it doesn’t feel like you’re spending a lot even if you’re spending thousands.
With laundry, maybe the customer wants to turn two loads into one but doesn’t have the cash or coins with them. When a customer is able to pay with their credit card, they have the option to spend more. This boosts your bottom line and reduces wear and tear on machines from people overloading them.
People spending more can be especially true if your laundromat or apartments are frequented by higher income earners in desirable metropolitan areas. These customers are price-insensitive and upping a vend could easily return a solid ROI.