The disciples wanted to know why Jesus spake in parables. Jesus spoke with the intent that His hearers would understand. When He spoke there were two different classes of people that heard Him; those who did not understand and those that did. Jesus wasn't suggesting that He was hiding the truth from the people. It is simply that some had closed ears and minds. His disciples were receptive and they heard His words and applied the meaning of His parables. So, if anything, the purpose of a parable was to teach a simple story and then made a spiritual application from the story. He tried to clarify things that were difficult to understand.