This seems to make sense. Could one benefit or purpose of standing still be that it gives the body and mind time to adjust and settle into a better posture and attitude? Time you wouldn't have (at least at first) if you were moving?
In particular, if people tend to rush around a lot in their lives, then maybe they go to the gym and their training is also rushed. They never escape that rushed and hectic feeling except maybe when they are tired and warm right after a workout. ZZ could in part be a corrective for that.
Simlar or different to basic concentration practices for sitting meditation?Master De Jian also explicates various natural breathing practices that combine these practices.
My understanding is that these can aid in the goal of bringing the mind to an "extreme point" of focus where all distracted thoughts are stilled.