They use Steam for money laundering:

• Acquire shady funds such as by scamming people into purchasing gift cards.
• Add said cards to their account.
• Have a "discounted Steam games" website
• When someone "purchases" a game, use the shady funds to actually buy it and transfer it to the consumer.
• The consumer is stuck with a game bought with stolen funds, while the reseller gets clean money.

Who buys stolen credit card numbers without a way to verify that they're usable? There are probably forums and other shady marketplaces where regular sellers can build a reputation and offer discounts on the card values so buyers are willing to take the risk. Not like they're losing their own money on the transaction.

Another mark.

They usually sell them for less than the amount on the card.