Piggy’s “specs” are used to start fires, and they are stolen because Jack wants fire for his tribe.
Piggy has worn glasses since he was three. It does not take long for the boys to see that they need some kind of technology to start a fire.
Jack pointed suddenly.
“His specs–use them as burning glasses!”
Piggy was surrounded before he could back away. (ch 2)
Piggy does not want them to take his glasses, because without them he can’t see. Ralph gives the glasses back. Later, when Jack suggests that Piggy is useless, Simon reminds him that they have to use Piggy’s glasses to start the fire.
Piggy’s glasses are broken when Jack hits him, angry because he is complaining that the fire was let go out.
He took a step, and able at last to hit someone, stuck his fist into Piggy’s stomach.
Piggy sat down with a grunt. (ch 4)
Piggy can barely see with one of the sides of his specs broken. He tries to threaten Jack, but no one takes him seriously.
When the group breaks, Ralph and Piggy are eventually the only older boys left on their side. Jack and the others target them, and ensures that they are the only ones with fire by stealing Piggy’s glasses.
They’ve got our fire.”
Rage shrilled his voice.
“They stole it!”
“That’s them,” said Piggy. “They blinded me. See? That’s Jack Merridew. (ch 9)
Eventually, this is what leads to Piggy's death because he and Ralph try to get the glasses back, and the boulder is pushed onto him.
The glasses are not just Piggy's lifeline, they are the boys' connection to technology and civilization. Without them, they are lost.