What is Piggy's contribution and his insight in Golding's "Lord of the Flies"?

In a moment of introspection in Chapter 5, Ralph reflects that "Piggy could think.  He could go step by step inside that fat head of his....Piggy had brains."  Ralph realizes that it is Piggy who is the rational element, the "specialist," of their little society on the island. Piggy is an antithesis of Roger "who carried death in his hands" and other boys who, in their disloyalty to Ralph as the leader, become hunters, giving blind allegiance to the more powerful.

In William Golding's allegorical examination of the inherent nature of man in "Lord of the Flies," certain characters represent qualities of human nature.  When Piggy first appears in the novel, he looks more adult than the other boys:  His hair is thinning, he needs glasses, he is heavy, he has medical problems. One of his first remarks is an astonished, "Aren't there any grownups at all?"  For, Piggy realizes that the adult world represents order.  He, then, seeks to bring this order by stressing the importance of building shelters and maintaining the rescue fire, and by summoning the boys to meetings with the blowing of a conch.  In Chapter 5 when the little boys express their fears of the "beast," Piggy replies, "We know what goes on and if there's something wrong, there's someone to put it right." And, in Chapter 8 when the boys are frightened by the beast on the mountain and Ralph feels "beaten" because they can no longer tend the signal fire without encountering this beast, Piggy wisely suggests the obvious:  They can build a fire right where they are on the beach.  He also perceives that Jack is a problem.

However, as the boys remain on the island with no adult supervision, Piggy becomes "in this context...an irrelevance," and other forces, those more savage and brutal (Jack's), supercede the rational.  When Roger, whose sadistic tendencies have no controls of law, releases the rock that splits the head of Piggy, all control and reason is destroyed.  Isolated from the others and fearing his life, Ralph

wept for the end of innocence, the darkness of man's heart, and the fall through the air of the true, wise friend called Piggy.