It has to do with a chemical reaction affecting the flavor of your recipe. Baking soda, in essence, neutralizes acidic ingredients. So if you're aiming for that buttermilk tang in your buttermilk biscuits, baking soda might neutralize it. On the other hand, baking powder might not be powerful enough to give your biscuits the perfect rise. So a careful balance of both leavening agents will give your baked goods a fluffy texture while retaining the tangy flavor profile.