The uses of usury

Last month my wife, a full-time mother with minimal income, walked into a dealership, paid a 10 percent deposit on a 2005 car with a credit card, and drove out an hour later having promised to pay the first installment in a month or so. It would have been quicker but she wanted the windows tinted. She always pays off her two credit cards every month, which acts like a magnet for an endless torrent of offers for more credit from banks and card issuers. One wonders what’s in it for the lenders