Robert Stein, in An Introduction to the Parables of Jesus give three reasons (read, p 33ff)...
1. To conceal his teachings from those outside (Mark 4:10-12)
"Time and time again Jeuss found in his audience those who were hostile toward him. The Sadducees saw in him a threat to their sacerdotal system. His attitudes toward their doctrine (Mk 12:18-27) and above all to the abuse in their role of administering the Temple of God (Mk 11:15-19, cf 14:58) were a direct threat to their civil and religious authority (Mk 11:27-33). ...Many of the Pharisees likewise saw in Jesus a threat to their own self-righteousness (Lk 18:9-14) and their religious leadership..."
"By his use of parables Jesus made it more difficult for those who sought to find fault with him and accuse him of sedition....The parables therefore concealed his message to those outside, but privately, after they were explained by Jesus to his followers, they became revealers of his message."
"Yet we must be honest and admit Mark 4:10-12...seems to say that Jesus withheld his message from those outside not only in order that they would fail to understand but in order that they would be unable to repent and be forgiven."
2. To reveal and illustrate his message to both followers and "those outside" (Mark 12:12)
"For the original lawyer as well as every reader since, the parable of the good Samaritan illustrates in an unforgettable way what it means to be a loving neighbor, and if one sought an example to illustrate the gracious love of God for sinners, where could one find a better one than in the parable of the prodigal son?"
"At times even those 'outside' did not and could not miss the point Jesus was seeking to illustrate in the parable.
And they were seeking to arrest him but feared the people, for they perceived that he had told the parable against them. So they left him and went away. Mark 12:12
3. To disarm his listeners (Luke 7:36-50)
"At times Jesus sought to penetrate the hostility and hardness of heart of his listeners by means of a parable."
OT example - 2 Samuel 12:14, "a perfect Old Testament example of this"
NT examples - Luke 7:36-50, "Here, in order to pierce through Simons hardness of heart and prejudice, Jesus spoke in a parable and hoped to reach Simon."
Luke 15 parables in response to Luke 15:1-2...
Now the tax collectors and sinners were all drawing near to hear him. 2 And the Pharisees and the scribes grumbled, saying, "This man receives sinners and eats with them." 3 So he told them this parable...