Shirshasana is called the king of the asanas and is thus considered the most important of all the asanas…
When we turn our bodies upside down, we are literally turning our world upside down. Turning upside down allows us to experience the advantage of different attitudes and ways to perceive. Everything we know as right side up, typical and normal is pulled out from underneath us. This disorientation requires us to draw from places in our psyches that we may not have accessed much before.
READ all: http://www.jivamuktiyoga.com/teachings/focus-of-the-month/p/inversions