In preparation for a class on preaching that I am teaching, I came across this helpful discussion of the purposes of sermon illustrations although, I think some of the points could be combined.  
- Inform and instruct
 - Explain and clarify
 - Can help the preacher connect and identify and identify with his people
 - An aid to memorization and recall
 - Helps to capture and retain attention
 - Motivate, persuade, and convince
 - Allow for mental relaxation
 - Help people see the immediate relevance of the biblical text for their lives
 - Personalize and particularize the general/universal truths revealed in the Bible
 - Make biblical truth believable
 - Create interest
 - Explain biblical doctrine and personal duty in an understandable and compelling way.
 
Daniel Akin, Bill Curtis, and Stephen Rummage, Engaging Exposition (Nashville B&H Academic, 2011), 199-200 (with some modification).