What is the purpose of the conch shell in lord of the flies?

The conch still exists and is still in its rightful place on the 'chief's seat', on the platform. Ralph choses to sit on the ground facing it, which probably symbolises an admission by Ralph that the democracy which the conch represents, and which elected him to the position of chief, no longer functions. Alternatively perhaps Ralph feels that, after taking part in Simon's murder he is no longer worthy of the title of chief. So I'd say that on page 171 the conch symbolises a lost ideal, an aspiration which was unachieved.