Overall, in my opinion, the load step is one of two things.
1) It is the stretching of the muscles to load the swing.
2) Re-loading the muscles from the original load in the stance.
If the 'load step' is 1, then it is comparable to winding up for the swing while the pitcher is in his delivery. This limits the plate coverage to only the area that the swing is loaded to. Secondly, it minimizes the time a batter has to read the ball out of the pitcher's hand to determine the type of pitch. If the load is 'fastball' then the batter is particularly vulnerable to off speed.
If the 'load step' is 2, then it is a redundancy. The sequence would become load in the stance, stretch and re-load again in the step. This is time consuming when the batter has to see the ball, read the ball, decide to swing and then launch the swing.
Since the 'load step' would also be weight shift, it would minimize the 'power' in the stance after a delay for an off speed pitch. The old expression is that a batter has no legs in the swing.