Good sermon illustrations do at least two things: they make key concepts in the sermon easier to understand and easier to remember. The primary way they do this is by turning that which is abstract into something concrete. As someone once said, they turn your listeners’ ears into eyes and help them see what you’re saying.
For example, I’ll never forget how Rick Holland compared the fleeting nature of human existence to the steam coming off the top of a cup of coffee (in Ecclesiastes), or how he described temptation as sugar-coated poison (in Proverbs 5). First I heard, and then I saw. And even now, more than ten years later, I still understand and remember.