These are a collectible in GTA Online, added with the Diamond Casino & Resort update. They are scattered throughout San Andreas, and for collecting them you receive GTA$, RP, and other rewards.
Playing Cards are a collectible in GTA Online, added with the Diamond Casino & Resort update. They are scattered throughout San Andreas, their locations are shown on the map below. You can track how many Playing Cards you have collected via the Inventory section of the Interaction Menu.
The first five cards give you 100 chips each, each next one gives 50 chips more than the previous. The total amount of chips for collecting all the cards is 66,650. Also, after collecting all 54 cards, you get a unique deck of playing cards on the private table in your penthouse, a special High Roller outfit, a new decoration for the penthouse, and an exclusive reward in Red Dead Online. Furthermore, each collected card gives you 2000 RP.