Piggy provides the voice of reason in the book and tries to establish order among the boys. He's intelligent, providing answers to help the boys survive and suggestions to help them be rescued. His looks, behavior, and beliefs resemble those of an adult, reflecting a more mature attitude and greater wisdom than the others. He frequently asks "what grownups would do" when confronted with situations that the children don't know how to handle. He sees the importance of building shelters and moving the fire to the beach. His glasses are the tool to build the fire. Only he understands Jack's hatred of Ralph.
Unfortunately, for all of Piggy's adult-like behavior and reasoning powers, he's a short, fat, awkward, asthmatic kid on the outside. His mental acuity is a contradiction of his physical weakness. "Mocking him makes the others feel cheerful." He's unable to survive in a society where reason is replaced by irrationality. Perhaps his physical appearance is symbolic of a child's tendency to rebel against an adult society of rules and order or a reminder there are no adults to save them.
At the end, Ralph cries for "the end of innocence, the darkness of man's heart," and for "the fall through the air of the true, wise friend called Piggy." Piggy is more adult-like in his appearance and behavior, but he also has the innocence of a child's soul.