Winning the card tournament in Treno, winning 3 consecutive games will gain you a Rebirth Ring. This is part of the games course and you cannot escape it.
Having the Namingway card (#98) will allow you to change the name of your usable characters if you talk to a guy at the left hand side of the library in Daguerreo.
Having lots of different types of cards will raise your treasure hunters rank, another side-quest in its own right, plus you can see your own card collectors points in the card menu.
I consider having the ultimate 1700 card collector's rating a reward in its own right.