Aesthetics and also it can feel different on your foot. Different pedals may keep your feet bent at a different angle when hovering over them and also may provide you with different grip depending on the design

. IE my stock rubber pedals were bending my ankle up to an incomfortable angle. Different depths of the pedals can also reduce the distance travelled by the foot when pressing the pedal(not by a lot though), not to be confused with the pedal travel distance. Also different pedal widths may
help you if have a tight space in which to do your footwork in

. So overall there are a few factors to consider when buying new pedals.