Zhan Zhuang Is a Gift from Evolution of Ancient Hunting Human

Standing Meditation (Zhan Zhuang) is developed during evolution of human hunting in the field for the preys. It is not a chance that Zhan Zhuang is the core of internal martial arts like Yiquan. Zhan Zhuang develops the attacking instincts and the internal energy needed for the ultimate attack against a prey. Lets explain the mechanism. The ancient hunting human (like predators) needed to keep an attacking position in which the relaxation and readiness-to-attack were the required conditions for a successful predator. So the evolution equipped us with a mechanism to generate energy in this standing position in order to be successful as predator. The generated energy in this standing positions is easily shaped by the intent of the ancient human (or predator) and therefore can be easily charged  for the ultimate, fast attack toward the prey. The intent is just the catching the pray.
The intent is the key in this energy discharge. But, additionally, the ancient Taoist masters noticed that this energy not only can be used for fighting but also can be used for healing.

So the standing position increases relaxation in such a way that you as predator will not frighten the the prey. It also increase the fluid internal energy which is available to be used instantly to hit the and catch the prey.

In every hunting position the legs are bent the same as Standing meditation.

This practice increases the efficiency of nervous system for coordination of a successful attack against the prey. Therefore for long standings it strengthens the body's Nervous System and develop the coordination required for effective attack which then was discovered and used by martial artists.