its needed to maintain good balance with current level of food constructions and garrison size. Peace time, allways garrisons amount so that, you get at least +1 food, if none of food constructions is done, you cant maintain garrison pretty much anything. Wars will destroy your economy, as enemy raid your villages and kill your caravans and civs, so food production comes crashing down and so will your garrisons. In your city you can help it by selling yourself huge amount of food there. Its pretty unbalanced at the moment