The Power of Symbols
Storytellers have always used symbols. Even the most ancient texts contain rich symbolism. So do tales predating the written word by millennia.
At first it seems counterproductive to wrap ideas in layers of metaphor. What's easier: saying, "Being too single-minded can land you in trouble," or writing a 635 page book about a guy chasing a whale?
So why do human beings like our messages delivered via symbols? Whatever the reason, it's ingrained deep in our nature. There's no denying that concepts encoded in symbols enjoy far wider distribution and have much longer shelf lives than dry, straightforward discourse. It's a safe bet that the number of people who could tell you one of Aesop's fables is greater than the number who can recite a given passage from Plato's Republic.
Symbols are powerful tools to convey meaning. Contrary to the example above, they can do so quite efficiently. A yellow sign with a moose on it prompts you to drive cautiously better than one saying, "There are moose nearby. They will probably be crossing this street at some point, so you should slow down unless you want your car totaled."
Though symbols are effective, they are best used with a light touch. The paradoxical nature of symbols dictates that their effectiveness increases (at least in literature) with their subtlety. A scruffy ranger who wins a kingdom through trials and selflessness conveys the idea of messiahship without audiences necessarily being aware of what he signifies. On the other end of the spectrum, a talking lion who dies and is resurrected is such an overt allegory that it almost defeats the purpose of using a symbol.
Speaking of which, the Christ figure is probably the most enduring and ubiquitous symbol in all of literature. The same holds for pre and non-Christian societies. Whether a writer thinks that the symbol's content is true or not is irrelevant. There's something fundamental to the human condition that makes most people want it to be.
I've been devoting a lot of space to writing advice lately, but all I can really tell you about using symbols is that 1) you'll do it whether you mean to or not; and 2) resist the urge to explain the symbols you use. As Dean Koontz explains, stained glass windows don't have subtitles.