Just wondering...

It isn't "keeping the weight back" that helps as much as leading with the hips rather than the upper body. When the upper body gets ahead of, or even with the hips early on it can cause problems, so pitchers are given the cue of keeping the weight back. The current fashion is continuous movement forward through the pitch, but there are enough pitchers with a pause or who actually keep their weight back and are successful that it isn't an absolute.

If a pitcher can move forward continuously through the pitch while leading with the hips and staying in sequence then that is probably the best approach.

Excellent reply from CADad.

Unfortunately, coaches who give the "stay back" teach don't explain it that way so pitchers interpret it as meaning to keep the whole body back. In fact, I'm not sure all such coaches actually mean "keep the upper half back" or even understand the difference. Keeping the whole body back can limit how much energy you create to put into the ball, it can lead to timing problems, it can make you slow to the plate, and it can cause you to throw with too much arm. Don't confuse the "feeling of throwing harder" with "throwing with too much arm".