6 Answers

BY WHOM?   Looking at the questions you have asked so far at QA, my guess is you want the answer to be JESUS CHRIST.  IN MY OPINION, He did use parables, for the reasons I have cited in my other response.  
For what CAUSE???  Again, see my other response.

"I want you to figure this out on your own.  I'm not just going to give you the answer."
My guess would be parables and metaphors are utilized so the person hearing or reading them has the opportunity to make a parallel connection to some aspect of himeself.  They are subtle, but when someone takes the time to think and correlate, they can be more effective than having the true message spelled out in no uncertain terms. 
Sometimes a metaphor can express something that is difficult to put into words.   
A lesson learned by self-discovery can be much more effective, meaningful, and lasting over one that is handed to a person.  One might call them an opportunity for an "A-HA moment".

simple if it was plain and simple only the plain and simple would see it...it takes wisdom to solve an enigma, and a wiser man yet to seek it...

Parables and metaphors were spoken by Jesus to the simple people of that time,and they were understood by the people of that time,,the highly intelligent and scientific person does not take it on face value like the simple man,but rather confuses the issue,,When Christ said,"It is easier for a camel to pass through the eye of the needle,than a rich man to enter the gates of heaven."the people of that time understood it as clear as day,,but the people of today have problems with it,because we do not park our camels outside the hole in the wall..>>>>>>>><<<<<<<<..

 God bless  ;-)