Simon is significant to the story for a couple of reasons. First, Simon represents goodness. Not simply acting good, though. Simon is innately good and kind to others. He does it because it is right, not because it is forced upon him by rules or adults. Knowing that about Simon makes his death an important turning point in the story. His death signifies the final remnant of good leaving the boys. Once Simon is dead, the tribe of boys breaks down into a mixture of anarchy and dictatorship. It's a group of boys in a power struggle instead of a group of boys working together for survival.
The other reason that Simon is significant to the story is that he sees the evil (The Lord of the Flies) for exactly what it is. Simon learns that the monster isn't a physical enemy. Instead, the monster is their inner evil coming to the surface. It's the evil that exists within each person.
“You knew, didn’t you? I’m part of you? Close, close, close! I’m the reason why it’s no go? Why things are what they are?”