Simon is the mystic of the group. That means that he is the one who understands, before anyone else, what the problem on the island is - specifically, that the beast on the island isn't a tangible entity. The beast is the evil within each one of the boys. He intuits more than any other boy and he needs to go off by himself because the revelations come to him when he is by himself. He must be away from the others because they emit noise through their arguments and frustration through their negative energy. Chapter 3 is the first time that Simon goes into the jungle by himself and thinks. Chapter 7 is the last time. It is in chapter 7 that Simon has his converstation with the Lord of the Flies - the skull of the dead pig that Jack stuck on a stick as a sacrificial offering. Here he fully understands what is amiss on the island. Appropriately, as he goes to share insight with the other boys, he is killed by those boys in their frenzy.
Simon is also more physically fragile; in the past he has suffered from epileptic seizures. He needs to get away by himself at times to take a break from the building tension among the boys. It's his way of sorting things out:
Simon, for all his sensitivity and fears, knows that the only way to deal with fear is to face it. When no one else wants to climb back up the mountain after seeing the "beast from air," it is Simon who proposes just such a climb. "What else is there to do?" he reasons.
Simon must periodically retreat to the candle-budded trees in the forest to restore and maintain this clearsightedness.