Sure, people know that they shouldn't spend more than they make and --Oh look! There's a sale today at the mall; I have a credit card and it's a good deal, brb.

It's about discipline more than it is being good with numbers. Sometimes I would imagine it would be hard for people to look at their bank account and see enough money in it to purchase X, so they use their credit card to purchase X only to spend the money in their bank account on something like groceries later.