Explain the importance of Jack's tribe stealing Piggy's glasses?

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.