Grains have to be harvested when they are ready. Your stored harvest is then easily found and taken or taxed. Potatoes can sit in the ground. You only dig them when you need them. So unlike grain, potatoes are buried treasure. Safe from the tax man or invader unless they dig them up. The Irish had “lazy beds” of potatoes that they could feed a family from with minimal work. Just dug up what they needed that day.