Both makes it better.
Baking powder is:
1) Bubbles when I touch acid (baking soda)
2) Bubbles when I get hot (cream of tartar)
3) Anti caking agent.
The proportion of baking soda to baking powder is not ideal for the fluffiest pancakes, so boosting with more baking soda helps.
More baking soda also helps with maillard reaction (golden brown texture).