Basically, someone stealing CC numbers will need to find a way to monetize them. This can be done for instance by printing fake cards with the stolen info, using them to buy goods and then sell those goods again. For someone who only wants to do the hacking part (not the real-life monetizing part that includes shopping), selling the stolen CC no's to people who do that second part is a good way to make money.
I'm sure there are many more options to monetize CC numbers. Since they're a widely accepted payment method, you can be quite creative with them :-)