Just like description, examples are often used as a concrete way of illustrating a main point, rather than just trying to explain it in an abstract sense.   Examples are incredibly useful tools in writing, because they give the reader a context for a broader, more general statement.   If the thesis of my essay (or the main point of my short story) is that big dogs make the best, most loyal, faithful, gentle and obedient pets, then I'd better have a concrete example or two to back up that statement.   Often you can develop an example by using description.   I could describe the gentleness, faithfulness and obedience of my last dog Molly, a 120 lb. Rott/mastiff mix, in very concrete terms, and it would help to support my main idea that big dogs are wonderful pets.

     So you could say that the main job of an example is to back up a larger generalization.   If I claim that genius and madness go hand-in-hand, I ought to provide several examples as a way of backing up that statement.   Not only does it support my main idea, it can help reinforce the reader's interest.   If none of your ideas are grounded in examples - if they exist only on the general or abstract level - you are bound to lose your reader quickly.   What examples to include, and how much detail to use for each, will (of course) depend on what the generalization is, and who the audience is.   I might need to include more than one or two examples in order to convince a reader that big dogs make better pets than little ones.   Just one experience isn't likely to change the mind of anyone who doesn't already agree with me. I might provide examples of magnificent big dogs that I've known, but my point might also require providing examples of little dogs that were less amicable and friendly.   Notice again how the narrative, descriptive and example techniques might function together.   I could tell about the time my daughter was viciously attacked by a 5-lb. miniature Doberman, and put that example right next to a description of how she and Molly, who was ten times her size, used to curl up together to take naps.   In this case, the story and the description work to reinforce the example, which in turn works to support the overall thesis.