Robert Stein in Jesus the Messiah gives some explanation of Jesus' use of parables (pgs 124-125)...
Parables are "extended metaphors, which tend to teach a basic point. At times, however, the details of a parable may bear allegorical significance (Mt 13:24-30, 36-43; 22:2-10; Mk 12:1-12)."
"The parables were particularly useful for Jesus as a teaching device. Parables tend to disarm the listeners, for the meaning of a parable is often driven home before they can resist the point being made." (2 Sam 12:1-4, 7; Luke 15:1-32)
"Parables were also an effective way for Jesus to introduce potentially dangerous teachings. To talk about the arrival of the Kingdom of God naturally raised concerns on the part of Pontius Pilate, the Roman governor. Yet the statements that the Kingdom of God is 'like a mustard seed' (Lk 13:18-19) or 'like yeast' (vv 20-21) wer sufficiently enigmatic that the political authorities judged them harmless. Through his use of parables Jesus could speak about politically sensitive issues. As a result, thouse outside his circle of followers could 'listen, but never understand...look, but never perceive' (Mt 13:14). but to those within the believing commjunity such teaching were explained."