If one knows the true name of a soul, be it that of the foulest daemon of the warp or the most stalwart defender of the god emperor. then one has ultimate power over that soul if one possesses the will to harness such power, matters of this nature are of pivotal importance in the endless strife between the denizens of the warp and the Imperium of man. And so it falls to Grey Knights to keep the secret names of their enemy once discovered, it is one of their most powerful weapons in the endless struggle. With a daemons true name, one might imprison the soul of a daemon and stop it returning to the warp upon the destruction of its mortal body.
The duty of guarding these names can only be entrusted to the purest of the Grey Knights, for even simply knowing the true name of a daemon would destroy lesser minds.