Golding himself said that he was influenced by idyllic island novels such as The Swiss Family Robinson and Robinson Cruesoe. Though he had enjoyed reading these novels and sharing them with his family, something in him made him think the novels were too romanticized and false. So he chose to write his own island novel to show what he believed might really happen to a group of boys stranded on an island.
The island also allows Golding to create a microcosm, literally a "small world," to show how people behave. The boys on the island are completely cut off from the rest of the world, and gradually they lose the trappings of civilization that they had brought with them to the island. The microcosm--the isolation--allows Golding to show how different individuals with varied human traits would operate in a vacuum, especially as they revert to their elemental human selves. All the characters and the various settings on the island are, in fact, symbolic of what happens in the larger world--what we are all like. In his afterward, Golding alludes to Conrad's Heart of Darkness. Golding uses the island setting to study humankind's "heart of darkness."
A more in-depth analysis of this novel is available on e-notes at this link: http://www.enotes.com/flies/introduction