It's just human nature to adopt views of ourselves and the world around us that make us comfortable...or rather, don't make us uncomfortable. So without some way to cut through the cozy mental couches we tend to rest on in our minds and hearts, we live with and through illusions, comforting ourselves with half-truths.
Jesus wasn't about illusions. He would burst anyone's bubble--causing temporary pain in the interest of long-term healing. The Kingdom of God--what he came to announce and demonstrate--is a place of reality and so requires a ruthless honesty. He used parables to do just that. Break through the illusions to reality.
I suspect Jesus was pointing to...transformation in seeing and hearing when he said it took "parables" to subvert our unconcious worldview--and thereby expose its illusions, even to us. Parables should make us uncomfortable if we are really hearing" them. If we fit them nicely in our business-as-usual world, parables have not served their purpose. A parable is supposed to change our operative worldview and unlock it from the inside--so that we can see and hear reality correctly.
What we have done for centuries in the West is give people new moral and doctrinal teaching without rearranging their mythic worldview. (And all it does is) create legalists, ritualists, minimalists, and literalists, who always kill the spirit of a thing."
From Richard Rohr's, The Enneagram: A Christian Perspective