Food (like already pointed out in another answer) is just used by the villagers to be converted to gold coins and running out has no negative effects (other than less gold)
For each unique card you collect you get a bonus based on the feature it belongs to (+5% damage for mining, +3% resource gain for village, ...)
You can also equip and activate duplicate cards in features when you unlock their prestige to get the effects listed (different for each card) until you prestige again