The beast, in William Golding's Lord of the Flies, represents fear and the power of evil. The boys create the myth of the beast in order to explain the fears they possess given the many unknowns which surround them on the island. Essentially, the beast is created in order to allow the boys to rationalize the unexplainable.
Later the beast changes. As the boys come to accept the existence of the beast (their fear), they begin to try to fight it. At one point, although he had promised to fight it, Jack tries to "befriend" the beast (in order to illustrate the solidarity between the boys--given they all believe in the beast's existence).
The beast, therefore, does not actually exist. Instead, it inclusion is rather simplistic. It is meant to illustrate the fear which exists in each and every person, and, since it exists in each and every person, it acts as a symbol of solidarity between the boys.