Rats as a whole have some health problems such as tumors. Ethical breeders are trying to breed these unhealthy traits out of the rats. It is not realistic to think that this will happen in one, two, or even three generations, but eventually we all hope to have tumor free rats with stronger immune systems (and hopefully longer lives).
Looks are just a little added bonus. People would rather have a blue rat then a PEW (pink eyed white). I personally love PEW's, black, and agouti (brown), but not everybody does. So we breed these fancy colors as long as the family line is healthy and sweet to provide people with more interesting looking pets. Of course the rats are all the same, just the fur is different