They aren't buying them back for use, they are buying them back to find correlations and figure out where the breach happened. The site basically allows you to buy cards from 'bases'. Each of these 'bases' are just a source of cards so if all of the cards they buy were used at the same place (which only banks will have this information on where the cards have been used), then you know that the breach is there. Along with this, I bet they also buy cards that the owners don't know are even stolen yet which means they can cancel and reissue those owners new cards without anything ever being stolen. So it is a combination of finding the breach and to also stop their customers from having all the hassle of having stolen credit cards.

> Interesting that it's cheaper for the banks to buy back their own cards than reissue them during giftmas.

They are not "buying back" anything. The cards can be resold at will.