Jack uses the other boys' fear of the beast to bolster his own importance in the tribe:

"'Quiet!' shouted Jack. 'You, listen.  The beast is sitting up  there, whatever it is--' [...] 'Hunting,' said Jack.  He remembered his age-old tremors in the forest.  'Yes, the beast is a hunter'" (126).

In chapter eight, "Gift for the Darkness," the hunters and Ralph have returned from their search for the beast, and they have seen its billowing shape on the mountainside.  Jack uses the moment to play on their fears, pointing out Ralph's cowardice and lack of hunting ability as reasons why the other boys should choose him as chief over Ralph.  For Jack, the beast is an opportunity to make himself appear stronger and braver than Ralph; he uses the polarizing topic of the beast to draw attention to himself, capitalizing on the boys' fear of the unknown and dark jungle paths. 

Jack created the idea of a mysterious beast that no one has seen in order to make the members of his gang fearful. By instilling fear, Jack tries to make himself out to be a better leader than Ralph by offering his gang protection from the beast. By constantly alluding to the beast and reminding the boys of the existence of a beast, Jack can control them in two ways.  One, he can control where they go (and don't go) and he can make sure that they stay true to him for protection instead of defecting to Ralph's team.

Jack uses the beast in the same manner that real propagandists from history and today use scare tactics.  The beast represents a threat to the other boys and strikes fear in them.  Just like all scare tactics, it is not important rather a threat truly exists, it is significant for leaders to convince their audience simply that they need to feel fear.  This accomplishes two important goals of leaders like Jack.  First, Jack needs the other boys to believe him; so he creates the idea of the beast and provides just enough evidence of its existence in order for the boys to follow him blindly.  They rely upon him for information about the beast, and in doing so, they start to believe everything he says.  Secondly, because Jack seems to be the "go-to-man" when it comes to the beast, the boys view him as the solution to the problem.  This is the same tactic that Hitler and his propaganda machine used.  They convinced the German people that their economic crisis was caused by the Jews ("the beast" according to Hitler) and then convinced Germans that they alone could provide the only solution to this "dangerous, malevolent" threat.