At the beginning, the conch is used to call the boys together. It is a symbol of society, of civilization, and particularly of Ralph's desire to organize the boys into a meaningful group. It is given the authority to determine who gets to speak and rules are made that when someone has the conch, the rest of the boys are supposed to listen.
As time goes on, the authority of the conch begins to wane. It is overshadowed by the power of Jack and his hunters and their idea of fighting back against the beast or whatever else frightens them. Instead of organization, the boys begin to crave the violence and ritual of a more savage way of life.
The conch as a symbol of civilization is destroyed when Piggy is killed by Roger. The dream of organization and civilization is destroyed with it and Ralph realizes that his hope for the boys, his hope for rescue, will certainly not come the way he imagined.