The fact that the boys are stuck on the island is brought up quite often through out the book, mainly when morale is at its lowest and the boys think that they are stuck there forever. This is also used to point out that they do not have everything they think they need to survive, no normal everyday things they had and have no way of getting back to where they came from, thus causing depression, and cabin fever. These things alone are what cause a book to have a sense of realism.
Since on an island there is most likely very little food or shelter it creates the fear that the boys might run out of food to eat, and that they could run out of normal things, like clothes, since there is no way to replace them. To deal with this they have to create shelters and be able to ration the food they know they have with them, and that they can find on the island. This poses the problem of having to work as a team to get something done. Since there is a rivalry between Jack and Ralph it is very hard for them to work as a team, some of the boys are loyal to Ralph, some to Jack, and some just too lazy to work.
The boys are completely free from any civilization or outside influence and have the ability to do whatever they want, mostly without consequence. Because of this there is amazing disorder within the group and there are numerous problems of bullying. Since this is all on an island there are no people to tell the boys what to do except each other. This doesn’t do much to change the attitude of freedom because most of the boys don’t listen to each other, let alone respect the others. Because there is no one in control there is a great potential for a tyrannical dictator. This place is filled by Jack, who ends up being almost completely insane which sends him, and his tribe, on a rampage through the island burning it down.
All of these problems put forward by the island are why the boys are not able to get along, and thus the plot of the book is formed. I think that an island was used because of these problems that are mirrored with real life problem of the same type.