Well, the purpose of symbolism in any literary form is to elevate the everyday to the status of art. Symbols evoke meaning beyond the usual, literal meaning. Symbolism simply (and reductively) makes things more complex and interesting! How much more compelling is it to understand, for example, that the "green light" on Daisy's dock is symbolic of the "green light" she seems to give Jay, and how the symbol of the lake is akin to the distance between the two of them?
If you have specific questions about other elements in "Gatsby" that you think may be symbolic, please let me know.