In order to perform carding, carders such as Dumpsshop and Creditcarddumps, create bots to get access to gift card balances. Thousands of ecommerce websites can be a part of the process.
Carders, such as Dumpshop and Ccdumpsites, attempt to get gift card account numbers and request the balance from every card. When the balance is identified and it's not zero, it means the card has real money associated with it.
Carders then use these cards to make purchases. The beauty of using gift cards is that they are literally untraceable. Even if the cardholder can detect that their funds are spent, it's often too late.