Granary is a building that stores food to boost city health and its civilians.
Granaries store food to help increase a city's growth rate. They add additional health if your civ has access to Corn, Wheat, and Rice. and has double production if a civ leader is Expansive.
The Civilians tries to store their food to survive in a small house turned storage area for food that can be spend when times in need like famine and winter.