Baking powder and baking soda are both leaveners. However baking soda relies on acidic properties in other ingredients to active (like brown sugar, lemon juice, buttermilk, vinegar etc.), while baking powder has everything it needs to activate upon the addition of water and heat. Eggs are a classic cookie leavener, and since these are eggless cookies, we need a little extra leavening action, so we add baking powder.