It just makes good sense that as our heroes fight the forces of evil, they should get better at fighting the forces of evil. So as the story progresses, the fights should get easier and easier. Of course, having an overly easy fight is just bad drama, so you have to consistently increase the threat the heroes face. This results in the Sorting Algorithm of Evil. The first villain you meet is the weakest, and the last is the strongest. As the heroes get strong enough to defeat their current enemy, a new enemy will emerge that forces them to reach another skill level. It would be an Anti-Climax if the hero defeated the Baddest Ass and spent the remaining time contending with not-quite-as-Bad Asses.
