Answer: The spectacles represent the boys' only means of obtaining fire through reflecting the sun's rays, and fire itself is symbolic of survival and rescue. Jack snatches the glasses off Piggy's face to create the fire, despite Piggy's protestations, and his dependence upon them.
Explanation: Also, because Ralph cannot let Jack have the glasses (power), he and his tribe go after them. It is in this "battle" or attempt to retrieve the glasses that Piggy is ultimately killed. In William Golding's Lord of the Flies, Piggy's glasses were stolen so that Jack's boys could continue to make and control the fire.
