Jesus parables is to sift out those who hearts were unresponsive. His illustration separated people, it revealed truth to those whose hearts hungered for it at the same time his illustration concealed truth from those with proud hearts.
In Luke Chapter 15 illustrations, Jesus stressed mercy by his words. Mercy and forgiveness are fundamental qualities of the true God and Jesus is the reflection of God's glory and exact representation. Those illustrations testify to God's merciful interest in sinners, painting a picture for us to imitate like Jesus his Son, did.