While I am much, much more concerned about function and reliability over cosmetics, I would gather that many concern themselves with a weapons looks because, well, lets face it...these are expensive, personal items. Thus, they are worth caring about. It is simply about taking pride in ownership. Of course, sometimes preserving a pristine finish is quite difficult (military use, for instance). But, I am assuming we are speaking of "civilian" pride here.
Of course, if the weapon is a rare "classic," the reasons for caring about the finish is a no-brainer.
Plus, lets face it, a crappy looking weapon may very well indicate a complete lack of care and maintenance (i.e., a potentially irresponsible owner). I would imagine nobody wants that stigma attached to themselves.