The stealing of Piggy's glasses shows the true dominating force Jack possesses. Scholars have suggested the capture of this fire starting tool is an allusion to the Greek myth of Prometheus stealing the fire from the gods and giving it to man. Also, without his glasses, Piggy is blind which further reinforces his character as being weak and afraid despite representing the intelligence of mankind. Also, Jack's tribe stealing the glasses is another way to show the development of the boys as they approach the downward spiral of savagery.
Taking the glasses was Jack's tribe's way of taking control over Ralph's group. Without the glasses, making fire was extremely difficult. To have possession of the glasses is to have possession of power on the island. Many of the boys know this and Jack's hope is that many will switch sides, strengthening his numbers.
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. (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.