Due to the constant overriding hunger of the daemon within the body of the possessed they were often unpredictable and prone to disobeying orders in order to slate the daemons blood lust. The creation of these teams was meant to quell, temporarily these urges making the possessed of the Alpha Legion far more disciplined and deadly.
Stalker Terror groups were often deployed for a week at a time in an enemy controlled area and allowed to run rampant. The resulting carnage would often sate the daemons need for death and destruction while at the same time terrorizing the enemy and sapping their morale.
During the battles for Guttar, Stalker terror groups were employed to keep the Imperials off balance and create a sense of dread across the area for all imperial forces assigned to the battle. The group excelled at ambush suddenly appearing in the midst of an imperial patrol wiping it out and leaving mutilated corpses for other Imperial forces to find.
Another tactic they frequently employed was to tail an Imperial patrol and silently pick them off one at a time leaving a handful of survivors to carry the tale of dread of being stalked across the ruins.
Furthermore the infiltration and use of the Stalker Terror Group often confused Imperial forces in efforts to hunt down psi bombers and other daemonic incursions. This is due to the possessed nature of the group who spread a daemonic corrupt psychic trail with their movements making it harder to identify psi bomber unleashed by the Stygian Empire.