3. Provide a dynamic stimulus for adaptation. By that I mean, heavy weights dictate low reps and moderate weights dictate more than low reps. Varying rep ranges are associated with specific adaptations. For instance - myofibrillar vs. sacroplasmic hypertrophy is one possible theory. Big words that simply mean different types of muscle growth.
Make sense?
I should add that it's not The Only Way of going about things. But it's how I like to structure stuff in general.