Why simply use existing free materials ? Because existing free shaders (in my knowledge), are not as realistic as i would like. So I focused my studies on real car paint, to recreate them as much as I can to make my cars looks real. The biggest part of a car shade is the metallic sparkles in the paint and the way the paint fade on angles and curves. Is it a magical product ? Of course not, but it can help and save a lot of time. It's not magical because in automotive project, the light is a huge part of the project (maybe 70%), so your realistic shaders without a good lighting won't be magical, keep it in mind ! But maybe you'll be able to get a nice render without a perfect lighting system with it !