In addition, the setting is important because it takes the boys outside of the very society of which the novel critiques. Only by extricating them from the flawed society and placing them in a paradise like the biblical Eden can the author assert that such flaws as violence and intolerance are innate, not societally born; however, this does not explain the evidence that they can be and often are societally influenced. You see, the boys in the novel had still been raised for several years in that society and by different parents with certain positive attributes and also imperfections. In order to truly prove the innate quality of violence in them, they would have to be untouched by society from birth, which is implausible. Regardless of the outcome, the author indeed directs his audience's thought toward the nature of man, which is one of the major themes of the novel.
The setting of "Lord Of The Flies" is important to the plot for several reasons. First, because the island is isolated, it forces the boys to create /re-create civilization on their own. Second, because it is so lush, it is like Eden; this is less a plot issue than a symbolic one, but it is important. Third, the practical side of the lushness is that the boys aren't killed immediately (say, by harsh winters). Nor do they have to hunt to survive. They can survive on fruit, at least for a while, and they have spare time and energy to get restless—and they choose to hunt. Finally, and this is more a blend of symbolism and history, there is an expectation of the island as an earthly paradise/vacation site. That's what the rescuing officer means with the reference to Coral Island on the final page.