If I take a step back then return & jump as high as I can will this result in a higher jump than taking no step back, both feet remain planted & jump? The answer is yes.
The reason is you are creating elastic energy by putting the body in motion, even though it is lateral into a vertical jump. So......If I step back in the windup (properly in line with the plate) generate energy, then transfer it to the delivery, I will have more forward momentum, a more explosive stride & be able to generate more velocity.
Why does a golfer take a back swing? Why not just start from the top if there is no benefit. The answer is that there is benefit. You are putting the body on stretch & generating power that is transferred to the swing. Same with a baseball swing. Why not just start stride early & start from the launch point? Because you would have virtually zero pop.
Pitching tosses this concept all out the window & the reason is because it is a more complicated move & you may lose command. You also have a tendency to "rush" as opposed to control the move. So everyone goes to the stretch or rocker step because it is simple & you can still throw gas if you have efficient mechanics. However, there is more there for the taking if you can sort out a dynamic full delivery.