This is at least a bit understandable... if something bad happens, a (trade, or a 'classic') war, some large catastrophe somewhere, you want local farmers with all the equipment, knowledge, fields and warehouses ready to take over. Yeah, sure, today maybe you have a surplus of potatos, more than your people want to eat, and still more expensive from imported ones from some other country, but subsidizing the farmers means that in such extreme cases, you at least have the infrastructure to grow potatos to feed the people at home.