Jesus could have used either. He wasn't obligated to be non-fictional.
Parables were an important part of Jesus’ plan to impart Truth to us. A parable is by definition a fictional story that is told with the aim of making a point of truth. It creates a word picture, which is an excellent tool for learning and for transforming people.
First of all, parables are easy to remember. How much of the book of Romans have you memorized? But if asked to tell the story of the prodigal son, you’d probably do a pretty good job, and remember the message.
Second, because they were easy to remember, a parable could more easily be passed along. Therefore, these little “truth-stories” could dispense this truth more widely.
Finally, a story drives home a message with greater impact than simply telling a message. That is because it taps not only into our mind, but our spirit as well. We can begin to experience something of what the prodigal son felt when he saw his father running to greet him. And God’s desire is that we are transformed at our core, which includes our mind as well as our spirit.
