The point of running it is so that one body can't just be kept to be a concourse killer at every event it goes to. If it's a great body, than fine, it can win a contest. But it's life should be limited so that it can't keep on winning time and time again. If you have to run it, then there will be a limit on how much effort you put in it as well, and what you can do with it.
For example, you could do a way better job making a car look cool if you added a whole bunch of details (cockpit, etc.), but this sort of stuff would raise your center of gravity and decrease performance.
Basically, the rule is to encourage race cars to be entered into concourse, rather than a car derived from a static model that can move, and to encourage innovation and new models in the contest, rather than the same ones time and time again.
Personally, I couldn't care less since neither motivation exists at our club level events, but that's the intent of the rules.