While the boys on the island are busy stripping naked to hunt pigs with sharpened sticks, there's still one symbol of advancement, innovation, and discovery: Piggy's glasses.
On the one hand, the glasses are a pretty simple symbol. They're intended for looking through, and looking = vision; vision = sight, and sight = a metaphor for knowledge. Piggy knows things the other boys don't, like how to use the conch, and the necessity for laws and order. When the boys take his glasses, he can't see anything. "Seeing" is Piggy's greatest attribute. It's the one reason the boys don't ostracize him completely; it's the one way he's useful. Without his glasses, he's useless—and the world he represents is useless, too.
At the beginning of their Outward Bound adventure, the boys think starting a fire is a great idea, but they're stumped about how to do it. Jack mumbles something about rubbing two sticks together, but the fact is the boys just aren't wilderness-savvy enough to do this. So, they rely on a remaining relic of their old world. When the glasses break, that's one more link to civilization gone. Check out how it's described:
The chief led them, trotting steadily, exulting in his achievement. He was a chief now in truth; and he made stabbing motions with his spear. From his left hand dangled Piggy's broken glasses. (10.296-302)
Dangling and broken, these glasses are being direly misused. They're no longer a symbol of reason and smarts; they're a symbol of just how far from civilization the boys have come.