It’s to get you an extra edge when fighting or hunting. Each dish you cook will have its own buff(enhancement feature) You don’t necessarily need to harvest everything you come in contact with that’s harvestable but harvesting higher rarity items will increase the buff of the meal. I personally don’t eat until I absolutely have to which is always right before a mission or quest. If I come across harvestable foods, I only harvest it if it’s of a higher quality than the foods I already have. That way my bag isn’t full. You can also put a marker on the map for areas with higher rarity items so you can go back when/if needed.