Using the same image from our knockout example, if we actually did print the dark gray circle on top of the yellow circle the color where they overlap would be slightly different (simulated and exaggerated in the illustration).
In some instances, overprinting may be desirable and the results better than attempting to trap the colors. Small amounts of overprinting are often less noticeable than the white space that might occur with untrapped knockouts and easier to do than trying to set a perfect trap.
