I never did play the card game any (well I played it once, hated it and never played it again)

So is there anything to gain that actually relates to the gameplay. I know you get better cards and such, but that dont' really effect the actual game does it?

If you get cards then they refine into items (if you learn the ability) If you get rare cards then you get even better items like:
-Chubby Chocobo = 100 Luvluv G's
-Bahamut = 100 Megalixers
-Squal = 3 Three Stars
they are me favourites there are loads but if you refine them you cant get them back

The main advantage of getting cards is to refine them (but you must have the refine ability learnt by a GF) in to different items, as Omnislasher said.

They are also a big help in getting the materials for upgrading weapons, but most people do just play the card game to have fun and see how many they can get.

the whole point of the card game was just to have something to do if u ask me, and refining them into useful items and u can get ur cards back that u've refined just win them off the SeeD members on the ragnarok.