Why did jack steal piggy's glasses?

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. (They used the glasses in combination with the sun to start a small fire.) Since they had allowed the fire to go out before, they needed a way to restart it; the glasses proved more successful than rubbing sticks or stones together to create a spark. The theft of the glasses also reduced Piggy--the most intelligent boy on the island--to a nearly-blind boy who needed help to get around.