The incline and decline biceps exercises' effectiveness lie in gravity. Additional gravitational push is applied to the weight when performing
the exercises at diagonal angles, increasing the stress on the biceps and therefore increasing the results from the exercises when performed
correctly.
Another bicep curl variation is the "correct curl" where the palms are positioned on the barbell at the width they would land into once
they are fully supinated. This variation heavily targets biceps with less involvement from your brachialis and brachioradialis muscles. -Wiki-