The conch serves a particular purpose throughout the story, changing as time goes on and conditions on the island change. At the outset of the story, it is the thing that calls the boys together from wherever they had ended up after the crash. It is also held up as the first "rule," whoever blows it must be listened to. These are boys that were used to authority figures and suddenly without adults or the rules they make and enforce, the conch stepped in to fill the void.
And though the power of the conch started to erode with small things and the boys getting tired of "meetings" and talking on and on, it loses its power more permanently when the boys split into two tribes and Ralph and Piggy's desire to have rules and order loses ground to Jack's hunters and their desire for hunting and fun and meat.
At last when it is smashed as Piggy is murdered, the last shreds of civilization also appear to fall away from the island as Jack and his hunters chase Ralph, intent on killing him.