A lot of it is about prestige, ego and middleman transactions. Never underestimate the human ability to perceive oneself as better than X. Or their willingness to pay the society to affirm it. A few years ago it was evident that Delta was out performing all of AMEX's other business by a wide margin. That had a lot to do with business travelers so the timing on the revaluation of Delta as a partner couldn't have been better. They got an increase in the percentage of CC business. As for the nuts and bolts, presales of skymiles and CC usage drives the money in. Most sales now include a hidden percentage for the credit card middlemen. This is why the mom and pop shops give a cash discount or charge a transaction fee for CCs. Multiply that over our customer base and all the purchasing they do with the Delta branded card and the numbers get big quick. At one time the Delta credit card was the most used CC available. I don't know if it still is.