According to author Ralph Ellison why do symbols probably appear in literature

"Symbolism arises out of action ... Once a writer is conscious of the implicit symbolism which arises in the course of a narrative, he may take advantage of them and manipulate them consciously as a further resource of his art. Symbols which are imposed upon fiction from the outside tend to leave the reader dissatisfied by making him aware that something extraneous is added." - Ralph Ellison