This a great question. The symbolic use of the conch is pretty powerful and pervasive. There are three instances where the conch is prominent.
First, when the boys are on the island (after the plane crash), there is general chaos and mayhem. Piggy and Ralph find the conch and they blow into it. This allows the boys to find each other and get together. There is order and civility. In this way, the conch symbolizes a civilizing and organizing force.
Second, this point can also be seen in another example. When the boys come together to talk and discuss, the one who holds the conch is able to speak. This action once again shows the ordering forces that the boys are capable of, which the conch represents.
As the book progresses the conch is destroyed. At the same time, Piggy dies and Ralph flees for his life. As the order breaks down into chaos, it is symbolized in the breaking of the conch.