The extra fat in sour cream helps to moisten the cake, thicken the batter, and make it rich in flavor. Use a full-fat variety with at least 18% milk fat.
Add half a cup of sour cream to the rest of your wet ingredients, mix everything completely, then pour the batter into your cake pans and bake according to the box's directions.
Sour cream is slightly acidic, with a lactic acid content of at least 0.5%. The acid activates the baking soda and adds a mild tanginess to your cake to counter its sweetness.
This acidity may also help your cake to rise and bake more evenly. It prevents the cake's exterior from browning too quickly as the interior bakes through.