Not every cow is as good as another cow. Some cows are sick and old, and others are healthy and young. Some wheat is good and other wheat is moldy or stale. So if a person trades cows for wheat, he might have a hard time arguing over how much wheat each cow is worth. However, money is "standard". That means one dollar is worth the same as another dollar. It is easier to add up and count money, than to add up the value of different cows or amounts of wheat.