Match the thread size and hex to your car (commonly 4 mm thread, 7 mm hex on 1/10 and 17 mm hex on 1/8). A serrated face resists loosening.

Vibration and hard driving can loosen plain nuts. Serrated or nylon-lined nuts grip better and stay tight longer.

Mostly, but they are a cheap way to finish a car. They work the same as plain nuts as long as the thread and hex match.