Having more farmland during a war would allow for a larger capacity to produce food to feed the army. As the infantry is focused on the battle, things like food and other essential commodities increase in their intrinsic value because of what it represents, in that food gives energy to troops and the troops need energy to fight and win the war and/or battles. Napoleon and Frederick the Great are both attributed with saying "An army marches on its belly". Meaning fighting needs fuel and that fuel for soldiers is food, thus the importance of having a significant amount of farmland.
