It is just about necessary to use a deserted island in order to make the points that Golding wants to make.
His main point is that people, left without civilization, will act in very savage ways. In order to make that point, he needed to have kids (not yet fully civilized) and have them placed in a situation where there were no adults to rein in the kids' impulses.
A deserted island seems like the most logical way to accomplish this set up. He could, I suppose, have imagined a nuclear holocaust, but this would not be likely to leave kids only.
Golding needed a place that could be believable but at the same time would place the children in total isolation. The island is in the middle of nowhere with no set bearings which will make it harder for the probability of rescue.
There has been nothing to spoil the island prior to man's arrival on the island. It is mankind who ahs brought the negative changes such as the scar on the earth, the cut limbs, the dead hog on a stick, and death and violence. He needed to be able to show the progression of evil against a backdrop of something pure.
Most people think of an island as a paradise. However, a person still needs to perform the same basic needs; drink, eat, and have shelter. He allowed for the things the boys needed to exist on the island, but he cost of attaining them would become great.
Golding wrote lord of the flies as a juxtaposed contriverse to 'Coral Island' which was a boys adventure book of the time. He mirrored the messages of this book which was also set on an island with three characters, two of them being called Jack and Ralph. Coral isand too reflects issues of democracy and power however proves to be much more light hearted than Golding's parody, which goes on to portray his message of the inner potential in humanity for evil and destruction (which is both compressed and caused by society.)
It demonstrates irony. They were on a island yet, horrible things happened. Golding is trying to how that nothing is always like it seems. He is trying to show that when civilization goes away, humans become savages.Since they are desserted on an island that is the perfect setting, since they wont be rescued or a while.