Perhaps a look at the book "The Coral Island" gives us some idea of what it is that Jack was so necessary in Golding's response to that book.  In "The Coral Island," the English boys are the civilized people in the midst of all kinds of savagery, of polynesians who practice infanticide and cannibalism to pirates who kill their own, etc.

Golding felt very strongly that the boys themselves were just as beastly or savage as anyone else and uses Jack to demonstrate this.  As the previous post says, Jack was a leader back in the "civilized world," and he uses much of what he learned as a leader there to effectively pull the boys away from the civilized leader, Ralph, and into his own tribe of hunters and killers and savages.

So Jack was a necessary element to show that "civilized" boys are no different in terms of their baser instincts than anyone else.