"...they can also symbolizes Piggy's contribution to the group..."
This quote from "Island Thoughts...", shows that Piggy's contribution to the group is his glasses. Since Piggy has asthma and is over weight, he can't physically contribute to the group. The contribution of Piggy letting the boys use his glasses to start fires, symbolizes how he is still involved with the boys' rescue efforts, even though he is not physically able to help them.