If the carbon dioxide created by the acidic ingredient and baking soda is not strong enough for the baked goods to rise, baking powder is also used to give the necessary lift because adding more baking soda will make the end product taste bitter/metallic. You don't want that right?
I know it's a lot of information to process. None of it actually matters if you are just going to follow a tested recipe as it is. These subtle differences and theories come into picture only when you are trying to develop your own recipe. Even then it's going to be a trial and error process because there are so many other factors which influence the end result of the baked good.