Collecting all 20 is required for 100% completion and the associated trophy. Quick Tips for Collectors
You receive $250 for each card collected, which helps with early-game weapon upgrades. far cry 3 data cab 10
If you're having trouble finding others, purchase the "Items" maps from any automated shop in liberated outposts to mark all collectible locations on your world map. Collecting all 20 is required for 100% completion