Perhaps we can find answers in the pages of the Christian bible itself. According to the book of Mark (4:11-12), it sounds like the function of these parables was to confuse his listeners so that most of them would go to hell.
4:11 And he said unto them, Unto you it is given to know the mystery of the kingdom of God: but unto them that are without, all these things are done in parables:So it is kind of a test. Fail it, and off to hell with you. Sounds like a great guy, that Jesus. Maybe it is a good thing he probably never lived.
4:12 That seeing they may see, and not perceive; and hearing they may hear, and not understand; lest at any time they should be converted, and their sins should be forgiven them.
Too easy? Maybe the function of the parables, written quite awhile after the period during which Jesus supposedly lived by authors who had never met him, was merely to be ambiguous enough that they would require interpretation. After all, if this stuff was made too accessible, there wouldn't be as much of a need for clergy, would there?