Food Geek Brian Geiger explains that it all comes down to acid. Baking soda needs some in order to activate and create the carbon dioxide needed to lift the batter, and you want to use only enough soda to neutralize the acid (since leftover soda in the batter tastes gross). Baking powder is actually baking soda mixed with just the right balance of another acidic ingredient, so you don't have to worry about acid in your recipe or leftover soda in your batter!
Now the reason why both soda and powder might be used is because you might have enough soda to neutralize the acid in a recipe, but not actually enough to lift the batter. In these cases, Geiger says that a little baking powder will give the extra lift needed to make the recipe perfect. You could use baking powder alone, but then your finished baked treat might taste too acidic.
Geiger says that you might also use both soda and powder when you want the recipe to taste a little tangy or develop a nice browned color. Baking soda is the key to both of these!
• Check Out the Full Article: "Baking Soda and Baking Powder" by Brian Geiger, Fine Cooking Issue #102