Well, top students may find that they aren't being challenged in a regular/normal unit, and take advanced units to suit their level of ability. Advanced units have the added benefit of a greater diversity of topics covered that are often more interesting, and in some cases provide alternative means of assessment - i.e. independent research on a topic of your own choice, instead of a generic topic the whole class does. In some streamed degrees like advanced science and nutrition, you are required to take a certain number of advanced units, and plus I reckon advanced units would look pretty good on your academic transcript.