From Bryan Chapell in Using Illustrations to Preach with Power (14-15):
The mind yearns for, and needs, the concrete to anchor the abstract. To say that illustrations aid the intellect, however, does not mean that they are merely a congnitive crutch. Illustrations are not supplemental to good exposition; they are a necessary form of exposition in which biblical truths are explained to the emotions and the will as well as to the intellect.
Illustrations will not allow mere head knowledge. They exegete Scripture in terms of human experience to create a whole-person understanding of God’s Word. By framing Biblical truths in the world in which we live and move and have our being, illustrations unite our personalities, our pasts, our present, our affections, our fears, our frustrations, our hopes, our hearts, our minds, and our souls in teh understanding of that which is divine.
They are integral to effective preaching, not merely because they may entertain or clarify, but because they expand and deepen the applications the mind and heart can make.
To illustrate Chapell’s point, if sermons were pancakes, illustrations would be more like the eggs, as opposed to the syrup and butter.