Past masters would often face life-or-death combat on a regular basis. The ability to survive combat depended on several skills: mental clarity, grace under pressure, internal power, speed, and agility. Zhan Zhuang is a type of Qigong that develops all of these skills in one stroke. Past masters discovered that, by standing still in various postures, they could cultivate many of the skills necessary for combat.
There are many postures, and variations of the postures. Below are the most common variations used in my school.