When I ask to pay by credit card, many merchants ask me “don’t you have cash?”. I tell them it’s none of their business whether I have cash or not. This kind of merchant behavior is rooted on the false belief that people use credit cards only when they don’t have cash.

Your question also seems to be based on a similar wrong premise.

People use credit card instead of cash for several reasons: Deferred payment (without cost), superior fraud protection and, of course, REWARDS. In another answer, I’d treated rewards as self-explanatory (Ketharaman Swaminathan’s answer to What is the advantage of having a credit card if you already have a debit card?).

In this answer, I’ll not get into the basics of rewards except to narrate an extreme example of this benefit.

A Chinese ex-cab driver turned billionaire bought the world’s costliest painting a year or two ago. I don’t remember the price but it was somewhere in the region of US$ 130M. He paid for it with his AmEx credit card. He accumulated so many reward miles from this transaction that he’s entitled to an unlimited number of free first class flights for his lifetime.