Plants generally capture water through their roots which typically spread evenly in all directions, water travels down through the soil until it hits a ledge then travels along its path (called percolation.) That means a rough measure of the amount of water is plenty to guage requirements for shallow rooted plants, like most food crops. An acre is a measure of area and a foot is a measure of length, combined those give the 3 dimensions to make volume. Water is sold by volume, and for agricultural purposes that is the measure it is sold by to farms (typically.)