When baking, do you ever wonder why sometimes they ask for baking powder and sometimes baking soda, or sometimes both?

The simple answer is acid

When your recipe has an ingredient with acid, you create the correct chemical reaction with just baking soda to produce the correct rise and taste

When your recipe does not have an acidic ingredient, you use baking power, which has cream of tartar added to produce the desired reaction. 

Sometimes there is some acid quantity in your recipe but not a sufficient amount so you use  combination of soda and powder

for a perfect result