There was a study done that showed people on average will spend on average 18% more every time when using a credit card than they would if they only paid in cash. McDonald's saw their in-store sales rise over 57% when they started allowing people to pay with a credit card.
Using cash has a psychological pain to it that using a credit card doesn't and people on average are far more frugal using cash only. Personally I would rather using that 18% we save and fly when we want and stay where we want and not worry about 'free' hotel stays or airline tickets.
I have also found that having cash has increased my buying power considerably. DP and I haggle all the time on high ticket items and have found that cash makes prices instantly decline.
So maybe you do not spend or think you do not spend more using a credit card, then good for you. That is how they come up with averages. There are those who spend a great deal more than 18% too and justify it because they "pay it off each month". They are folloing themseves on the deals they think they are getting.
When I order inside of McDonalds I use my CC. When I go drive thru I pay cash. Guess what. I order the same meal at both places. I get a 1% rebate when I go in the store.