In Chapter 8, why is the scene where Simon sees the Lord of the Flies important?

The scene where Simon sees the Lord of the Flies is incredibly important for a number of reasons. As a good, kind, and sensitive child, easily the most sensitive boy on the island, Simon is the one who believes that they all need to go to the top of the mountain and see the beast for themselves. "I think we ought to climb the mountain." When the other boys cower in fear at his suggestion, Simon simply responds with, "What else is there to do?" (184)  He believes in facing one's own fear. In fact, he's the only boy on the island willing to confront the fact that the real beast lies within them all.

Simon wanders off alone. The extreme heat and his thirst is described in great detail. After silently witnessing Jack and his hunters' brutal killing of a sow and the subsequent offering that they made of her head to the beast, Simon continues to sit in the heat staring at the gruesome totem. He can't look away from it and is quite literally mesmerized by it. The head is now covered in flies and is starting to bloat from the heat. Simon begins to hear it speak to him and proceeds to have an entire conversation about the Lord of the Flies' plan for the island, the boys, and Simon. The Lord of the Flies threatens him and warns him to stay out of the "fun" that it has planned. It urges Simon to return to the others and play, but Simon intends to continue up the mountain. As the Lord of the Flies continues to warn off Simon, Simon either begins to have a seizure or falls into a faint—it's a bit unclear as to which. 

This scene is incredibly important because only Simon is willing to go off and confront his fear. Simon believes that the beast is within them all, that it's simply a reflection of their own savagery and that the boys have created the beast. This conversation with the sow's head can be viewed as an extension of that very same idea. Simon faints or falls into a seizure when confronted with the levels of savagery that humans are capable of.  

Another reason that this scene is important is because it foreshadows Simon's death at the hands of all of the boys. All of the boys listed by the Lord of the Flies kill Simon—using anything and everything at their disposal: teeth, hands, spears, rocks, etc. Despite this warning, Simon continues on up the mountain to confront the beast.