The companies that issue them also charge the user to use the card. For any money that the user does not pay each month (the user's debt), the company charges a percentage of that amount in interest. If you owe the company $500 and do not pay it that month, the company adds from $6 to $8.50 to the $500. Then you owe from $506 to $508.50. The interest is added every month, so a bill that is not paid gets bigger and bigger