Buttermilk is acidic enough that it will tenderize the gluten in your flour, creating baked goods that are light and fluffy. When combined with baking soda, the acid in buttermilk helps with leavening—creating a higher rise. When recipes include a lot of acid, like lemon juice or buttermilk, it needs the addition of baking powder or soda for the chemical reaction, creating a lighter crumb. It's science—delicious science.