I feel that Jack uses the idea of the beast to control his tribe by assuring everyone that the existence of the beast was true. By saying this, the members of Jack's tribe will thus be willing to obey him and as I have mentioned in my earlier posts, be willing to cast aside all their discontentment about Jack's arrogant and barbaric behavior for the sake of their own safety.
The more the members of Jack's tribe believe in the existence of the beast, the more they are manipulated by Jack as he gains more and more control over his tribe. Also, to further boost his status as a chief, he then challenges Ralph to hunt the beast at night, weakening Ralph's role as a leader as Jack called him a coward when he had hesitated. When they hunted for the beast at night, the dead parachutist was then mistaken for the beast by both Ralph and Jack, "confirming" the littluns' fear regarding the existence of the beast and had thus indirectly enabled Jack to gain more influence over his tribe.
All in all, Jack uses the idea of the beast to control and direct the boys in his tribe by confirming the beast's existence and from there, control them by claiming that he would protect them from the "beast" if they obeyed him and followed him.