Although the boys insult Piggy because of his weight, he is rather useful. Use examples and explain why.
The boys use Piggy in a couple different ways. First, and most obvious, he is the one with the glasses that they use in starting fires. His eye glasses give them the power of fire, a symbol of civilized man. Just as eye glasses tend to represent the intellect, or the thinking man, in the story they are the source of a level of life above the primitive. When Jack and his tribe raid Ralph's camp in chapter 10, it isn't the conch shell the raiders come for. They come to steal Piggy's glasses so that they can have fire which elevates them to a higher level of existence and, inversely, lowers Ralph's camp to a very primitive level. The glasses are then kept tied around Jack's waist, like a tool hanging from a toolbelt, or like a trophy.
Piggy is also used in a far less obvious way - at least less obvious to the boys. His intelligence is used. From the very beginning when Piggy tells Ralph how the conch shell can be blown through to produce the loud noise to Piggy's observation that it is themselves they must fear in chapter 5 to his final confrontation with Jack and his tribe - Piggy speaks the truth. In some cases, the truth he speaks is slow to dawn on the others, but it does come to most of them. Piggy is smart and he is pragmatic. His truth is not always a pleasant one that the boys want to hear and so it is resisted, even by Ralph. Piggy's words, though, do slowly reach the surface and it is in part because he finally realizes how right Piggy was, that Ralph weeps in the end.