Piggy is an extremely important character in The Lord of the Flies. He is by far the most intelligent and clear-headed boy of the bunch. He is a rational, scientific thinker. In other words he sees things clearly, and has an understanding of how things work. The glasses are symbolic of this role Piggy plays. It's a bit simple for a symbol, but the glasses literally help Piggy see things better, and figuratively he sees in a way that the other boys don't. Seeing the world the way Piggy does is what gives him power and why he is useful to the rest of the group. For example, early in the book when all of the boys are confused about how to start a fire, it's Piggy's sight (and his vision tool) that allows the boys to focus the sunlight and make fire.
So later in the novel when Jack takes Piggy's glasses and breaks them, it's symbolically showing that Jack has completely thrown away the clarity and insight that Piggy brought to the group.