This post contains spoilers for various Mists of Pandaria quests.
Alone among all pandaren, the Shado-Pan know the true horrors that lurk beneath the gentle earth that bears the crops. The Shado-Pan stand watch against the hordes of the mantid, give battle to the yaungol, and most importantly stand against the Sha. And in so doing, they must make a terrible sacrifice. Each member of the Shado-Pan is hardened by the trials she or he must endure, but not merely in a metaphorical sense. The horrors of war against the Sha reach into their very being, so that they alone among all pandaren are carved and cut away by the Sha and their presence. Their very souls are scarred by the battles they endure against creatures of pure hatred, fear, anger, doubt, despair and violence. In essence, the Shado-Pan endure this great horror so that the rest of Pandaria might be spared it.
Make no mistake - the Shado-Pan serve the function of an army for a nation with no ruler. The people of Pandaria are defended by Shado-Pan manning the Serpent's Spine, standing fast against the mantid and the yaungol, but that is the least of their duties. Their true vigil - their endless watch is against the Sha and their minions. The Shado-Pan see the hand of the Sha in the hasty march of the mantid across the Dread Wastes and Townlong Steppes and the driving of the yaungol into Kun'Lai Summit. And it is the Sha that they dedicate themselves to stopping, for they know all too well the painful effect of the Sha upon the mortal soul.
Indeed, they were created to stand against that very corrosion of the spirit.