Take for example your car's brakes. When your car is in motion, there is kinetic energy that must be dissipated to stop. When you apply the brakes, pads contact the rotors or drums creating friction, which causes them to heat up quickly. Air flowing over the brakes cools the brakes thus transferring the heat by convection and the car stops. Now, think about going down a mountain. You keep riding your brakes until they overheat. What happens? You can't stop and make the headlines.

Back in the '60s, there were a growing number of motor vehicle accidents from brake failures. The cause was traced to hub caps restricting the flow of cooling air and wheels without adequate air slots. Spoked wheels and those with big spaces can move a lot of air thus often improving braking. If you coat the brakes and wheels with mud, you may not be able to stop hard because the mud insulates the brakes keeping them hot instead of allowing cooling. I've had this happen many times when 4 wheeling.

Now, think about your rotors and drums. It used to be we turned them down on a lathe to extend their life. However, the amount of allowance you have is so marginal it usually is not worth it. Nowadays, the rotor is ususally replaced out of hand. Why? Because you need a minimum mass of metal to absorb heat from the friction of the brakes without distorting the metal, which would break contact causing failure. It takes a critical minimum mass of metal to stop a car.

Does this help?