White Masks are Theurgically raised corpses without Souls. They obey whoever raises them. They are named as such because they are normally given blank Faceless Soul Masks to wear.
The Gentleman uses them as couriers and fighters, though they are not suited to fighting. In a fight, they rush the enemy all at once, either dying in the attempt (and showing their handlers how the enemy fights) or they kill the target. Either effect is useful to the handler.
Hive Communes do not normally use White Masks as there are better uses for the human body after its death.
White Masks have lungs filled with Mist. Upon their death, the Mist is released into the surrounding area. If a Theurge has created the White Mask, the Mist can then be used for Theurgy, giving the White Masks extra purpose.