When you use baking powder in frying foods the baking powder helps make the food you're frying more crispy.
Such as when frying chicken and you want the chicken fried crispy then you would put the chicken in some baking powder.
The baking powder makes and creates tiny air bubbles on the surface of the battered chicken or meat when it is placed in hot oil.
The bubbles that are created from the use of baking powder expand the surface area of the batter which in turn break up its thickness, which results in a lighter, crispier chicken or other fried foods.
This is how KFC and other places make their chicken so crispy.
If you want crispy chicken at home then you would use the baking powder while frying the chicken which in turn makes the chicken crispy.