Baking soda is added to buttermilk to create carbon dioxide bubbles in baked goods. Baking soda, or sodium bicarbonate, is an alkaline chemical leavener that reacts with the lactic acid in buttermilk. That acid-base reaction releases carbon dioxide gas, which expands in the oven and helps batters and doughs rise.
This is why recipes using buttermilk often call for baking soda rather than relying only on yeast. Buttermilk also contributes moisture, tang, and tenderness, while its acidity can help soften gluten and affect browning. Classic examples include buttermilk biscuits, pancakes, soda bread, and some cakes.
A common mix-up is baking powder. Baking powder contains baking soda plus one or more powdered acids, so it can produce gas when mixed with moisture and heated. Baking soda alone needs an acidic ingredient such as buttermilk, yogurt, molasses, lemon juice, or vinegar. Cream of tartar is an acid, not the alkaline ingredient that supplies the reaction's base.