GREAT question, reader Vanessa. A lot of people wonder why some recipes call for both. If you consider that baking powder is really baking soda (combined with some tartaric acid and corn starch) the answer becomes a little clearer: sometimes you just need more “pop.” Adding a little baking powder to a recipe that already contains soda and an acid like buttermilk will provide it, since it’s a pre-made reaction in a jar. Why not just add more soda and buttermilk to your pancake batter? Simply because a small amount of soda needs a fairly large quantity of buttermilk to react with. Much more liquid in the recipe and your family will be eating crêpes for breakfast.
I’ll add that in many baking circles it’s dogma that acids and bases must always be perfectly balanced. Hogwash. It’s true that you don’t want a whole lot more baking soda in a fatty preparation like a cookie dough or a tea bread, but a little extra alkalinity will make either one appetizingly brown on the outside. Likewise extra acidity can be a boon to cake or muffin batters since acid helps the egg proteins set up faster. Why is that an advantage? Well, let’s say you have an idea for the ultimate cherry chocolate-chunk muffin, except your fruit and chocolate pieces keep sinking to the bottom of the muffin molds. Extra acid in the batter will increase the batter’s viscosity, holding the big inclusions in place until the muffins are fully baked.