If you have the potential for debt (with a credit card you do, with cash or debit card you don't), then in a bad month you add on stuff that you need. Or in a stupid month you add on things that you don't. Then you have a balance you can't pay off. If you're using cash/debit, you can't spend money you don't have yet.

For many people the credit card is a way of buying things with next week's paycheck. Yes, they can normally pay it off, but not always. They're just against the edge of debt by living off future earnings.