"Lord of the Flies" depicts the interactions of a group of young boys that have been marooned on an uninhabited island after their plane crashes, killing all the adults. The boys attempt to govern themselves, but personal conflicts, as well as human nature, begin to strip away the veneer of civilization from their behaviors and attitudes.
Piggy is an overweight, asthmatic boy with glasses and a tendency to think scientifically. He also shows a lack of common sense and social adjustment, traits which quickly mark him as an outcast, and an easy target for the mockery of the other boys. In literary terms, Piggy is a carefully-designed symbol for civilization and its weaknesses; without rules and punishments that can enforce them, civilization has no power, just as Piggy has no power to defend himself. By being overweight and asthmatic, Piggy represents the detrimental effects of a life without conflict; Piggy can afford to be overweight and asthmatic because the structured nature of civilization will care for him. Finally, his glasses sum up this symbolism into a single object; they represent human technology, the care that civilization provides for the helpless, and the "order" that civilization and science bring with them...all things that the island lacks.
Piggy's glasses become important to the other boys for the simple reason that they can be used to create fire. Jack's clan decides to steal the glasses for this purpose. While this touches off the final conflict in the book, it also represents the boys' final descent in to savagery. By stealing the glasses and using them as a tool, they will symbolically strip Piggy of his most important connection to civilization; we could even push this so far as to say that they will steal his right to live, as we can assume that he won't last long without his glasses. Not only do the boys commit a simple act of theft, they reject the rights and independence that civilization have given Piggy, and thereby reject civilization itself. Thus, the glasses are important simply for their ability to create fire, but they are important to the plot and its message because of what they represent.