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.