Believe they're for gambling (possibly cheating, you know the whole hidden cards up your sleeve routine
). I received a quest, believe it was in Port Royal, in one of the taverns. I had to win 150 via playing poker. Supposedly it rewards you with a card and the chap said not to use it on him. I did not, however, receive any cards.
Maybe not implemented yet?
I'll run the quest again and see if I actually get a card.
Stay outa jail!
-SH