Why do Ralph, Piggy, and Samneric go to Castle Rock in Lord of the Flies?

In chapter 11, Ralph, Piggy and Samneric (the identical twins, Sam and Eric) go to Castle Rock to retrieve Piggy's glasses. The glasses are usually used to light the fire, and Jack has stolen them. Piggy pleads with Ralph to get them back because:

I can't see no more and I got to get my glasses back.

Having the fire—the last remnant of civilization—is important to these boys because without it they can't send up the smoke signals that will, they hope, attract a passing boat that could rescue them.
The boys bring the conch shell to Castle Rock, wishing to remind others of Ralph's former authority. They are frightened and demoralized by how events have unfolded but are willing to take the risk of approaching Castle Rock.
Once there, they meet Jack's followers, painted as savages, and Ralph confronts Jack, accusing him of being a thief. Ralph demands the glasses back.