The first is clarity. A good illustration clarifies what seems mysterious or obscure to the listener.
The second objective of illustration is motivation. It also helps the listener appreciate the relevance of a particular point.
An illustration starts out like a picture, bringing clarity to a truth. It allows the listener to “see” what’s being said.
Then an illustration turns into a mirror, allowing the listener to gain “insight” into how this new truth affects him or her.
Finally, the illustration becomes a window, and therefore provides “vision,” transforming the new truth into mental images that prompt the listener to envision the world.
Adapted from Saying It Well