The hardener is a catalyst that make your paint cure or "harden", if you didn't use it your cars paint would never properly dry. Reducer is to reduce the viscosity of your paint. Not many spray guns can handle thick paint without it coming out like "spider webbing". Reducer is to reduce the viscosity of the paint or to make it thinner so it can be applied as smoothly as possible. Its also used to clean paint off of things after application.
Its also good when you didn't buy enough paint so you reduce it down to cover, hehe