This is a good question.
There are two groups in the Lord of the Flies. One group follows Jack, and the other follows Ralph. As the novel progresses, the tension increases and Jack gains the upper-hand.
In chapter 11, there is a showdown that takes place. Ralph blows the conch to get the boys together. He also hopes that the boys in Jack's camp will come over. Ralph and Piggy desire order, and they still want to be found. So, the signal fire is very important to them.
When Ralph asks Jack to give Piggy's glasses back, Jack refuses, and a fight breaks out. During this chaos, Roger shoves a rock down the mountain. Ralph dodges it, but the rock kills Piggy and the conch break.
Here Jack screams, "I'm chief." There is no need for a conch, because Jack has won. Moreover, there is not need for a conch, because the conch symbolizes order. What reigns now is chaos and savagery.