Class actually defines both the focus as well as the target area for successful play of a D&D game. There are many, many other game systems where things like Class, Race and Levels are dropped for a more free form approach. However, other games almost always have categoricals in them too. The nice part about D&D is it allows for quick understanding and play of the game without requiring the players to spend endless hours on tinkering with mechanics. Pick a class, a race, rolls some scores, and voilà, you have a fully functioning and balanced character to the challenges of the game world. Customization happens through play.