> Why spend money you don't have?

Classic mistake, using credit or specifically a credit card does not always mean you dont have the money.

> If you do have money, why do you need to borrow money you already have?

Do you have a mortgage? Do you also have any money in a bank account / savings / cash in your wallet? If so "why do you need to borrow money you already have"?

Indeed the mortgage example is even worse, because typically a morgage has interest paid whereas a credit card paid off within the month does not.

Because I get 2-5% of it back if I put it on a credit card and pay it off and the end of the month.