Some people get all the satisfaction they want out of model building just by building models. They have absolutely no desire to enter contests, or have their work judged, or win trophies, etc. Like you said, Chuck, the finished model is in itself the reward, and all the "reward" that person needs.
But some guys get their satisfaction by competing. It's just a different mindset. To the competitive types, building a model is only the first step in the process. Entering (and winning) contest is what revs their motors.
However one participates in the hobby is cool, as long as you yourself are getting your kicks. Personally I'm in the "no contests" camp... I have absolutely no desire to compete. But to the competitive guys, the competition is what makes the hobby fun for them... it's where they get their kicks. Some of us want (need?) the hardware, some of us don't. Either way is fine. Different strokes, as they say...