What an interesting comparison! Obviously, introducing elements of story into expository prose serves to make the ideas "come alive," or become concrete.

Take, for example, the first part of Invisible Man, where Ellison is trying to explain what he means by "invisible," and tells the story of the white man on the street who insults him. Ellison's argument is that black people are "invisible" in the sense that their subjectivity is erased or unrecognized; they don't "count" as people. In the story, Ellison confronts the man who assaults him; he is about to stab the man with his knife when he realizes that as an "invisible" person, he enjoys a kind of power over this man. When he reads about the altercation in the newspaper the next day, and it is described as a "mugging," he scoffs, "Mugged by an invisible man!" The story helps crystallize Ellison's thesis about black identity.

Similarly, Thoreau explains his thesis about the simplicity of country living through anecdotes about finding the farm. His purpose in writing is "to front only the essential facts of life, and see if I could not learn what it had to teach." In explanation of this, he tells the story of the Indian prince who was raised among woodcutters and assumed that he was one of them, only to have his noble birth revealed to him when he was found out by one of his father's servants. Thoreau uses this story as a way of getting at the life of ordinary people in New England, who think of themselves as ordinary but who lack the ability to "penetrate the surface of things."