Most cakes call for both physical fluffing which is accomplished by creaming the fats and sugars together, as well as chemical leaveners (baking powder or such). Is this needed. Why can't I just use baking powder?
Depending on what you are making, you can easily use just one of the mechanisms to create tiny air bubbles in your finished product. On one end of the spectrum are for example pancakes: you mix just enough to ensure that the baking powder (or soda) is distributed well enough, then bake. On the other side we have sponges that work fine with just a base of beaten eggs and sugar.
A rich pound cake is something like a middle ground: it will actually work even without chemical leaveners, provided the fat and sugar were creamed well, but the baking powder is like an extra “boost”. But you need the creaming step for a simple reason: You need the solid fat to be soft and dispersed enough so that the gas from the baking powder can actually lift the batter and enlarge the existing tiny air bubbles created with creaming. And I find it difficult to think of ways to make a batter without softening the fat, simply because you need to mix the ingredients properly.
And finally: The amount of baking powder that you can add is limited. Use too much and it will negatively affect the cake’s taste.