Manufacturers use brake hardware to help with component fitment, reduce noise and vibration and also help reduce component movement friction. Precise dimensions and smooth edge surfaces free of rust and well lubricated will allow the moving surfaces to glide across the slipper, hardware, backing plate and caliper, thus eliminating noise. Brake hardware is subjected to the same environment as the brake pads and rotors and should be replaced with every brake service.
Premium coated rotors are designed to prolong corrosion and maintain higher thermo stability, allowing for extended brake system longevity.
Rotor warping/runout can be due to improper rotor installation during the tightening (torquing) of the lug nuts.