This is an allegorical novel, which means the characters and things in it represent more than they are in actuality. Things such as Piggy's glasses, fire, and pigs are clearly symbolic; the same is certainly true of the conch shell.
Piggy is the first to spot the shell, though Ralph is the one who first puts his hands on it. Piggy sees it as more than a shell; however, Ralph just wants to blow it because he likes the sounds it makes. That simple act, blowing the conch, accidentally gathers the boys for their first official meeting since the crash. Ralph is even elected leader based on his possession of the shell.
Soon it is the symbol of order--whoever has the conch has the right to speak without interruption. Ironically, it works for everyone but Piggy. Later, as things on the island deteriorate, so does the shell; it is fragile and nearly see-through from the effects of the sun and sand. Jack blows it "inexpertly," and Ralph is unwilling to blow it because he's afraid no one will come.
Finally, Piggy treks to Castle Rock where Jack has established his tribe. He can barely see his hand in front of his face, but he clutches the conch in the hope that law and order might still exist. Instead, he is met with mocking and jeering...and stones. Piggy fell to his death, and "the conch exploded into a thousand white fragments and ceased to exist."
Piggy was the character who most represented logic and reason and order, so it's not surprising he held the conch in such high esteem. Once he and the conch are both gone, there is no hope for the rest of them...until they are rescued from the island, of course.