Why Parables?

The meaning of the word Parable:
A parable is a story told to illustrate a lesson, a fact, or a truth.

Have you noticed that when Jesus spoke to the crowds, He often spoke in parables?
Since almost no one understood the meaning of the parables, he would not elaborate their meaning to the crowds, but only to His Apostles, as He explained in Matthew 13:10-23,
"Why do you speak to them in parables?" And He answered and said,
" To you it is given to know the mysteries of the kingdom of GOD,
but to them it is not given."
Mark 4:10-12, Luke 8:9-10.

Why did He explain His parables only to His Apostles, and not to the crowds who gathered to listen to Him?
Since His physical bodily presence on earth was limited, it was to show to us that all generations would receive the knowledge of the mysteries of the Kingdom of GOD through the Church which He came to establish.

"Of this gospel I was made a minister according to the gift of God's grace which was given me by the working of his power. To me, though I am the very least of all the saints, this grace was given, to preach to the Gentiles the unsearchable riches of Christ, and to make all men see  what is the plan of the mystery hidden for ages in God who created all things; that through the church the manifold wisdom of God might now be made known to the principalities and powers in the heavenly places. This was according to the eternal purpose which he has realized in Christ Jesus our Lord, in whom we have boldness and confidence of access through our faith in him."
Ephesians 3:7-12

It was His commission to the Apostles and to their successors to go out and teach to the whole world all that He had taught them.

And he said to them, "Go into all the world and preach the gospel to the whole creation. He who believes and is baptized will be saved; but he who does not believe will be condemned."
Mark 16:15-16 
See also Matthew 28:18-20.

He speaks to His Church and His Church in turn speaks His Word to the whole world.
If you reject His Church, then you have rejected Him.
"He who hears you hears me, and he who rejects you rejects me, and he who rejects me rejects him who sent me."
Luke 10:16
There is the obvious meaning to that verse, and there is the not so obvious meaning that is not grasped by many. In the first six words is the promise of Jesus Christ that His Church would not, and could not, teach other than what He Himself has said, and that could only be the fullness of truth. Anything coming from the mouth of His Church comes from Him, and He is truth personified (John 14:6).
This is nothing new, as GOD always spoke through a human representative on earth, a 'father figure', be he Abraham, Moses, David, or Peter (Acts 15:7), to name  a few.