Why are catalytics used in car exhasts?

Why are catalytics used in car exhasts?
When a gasoline engine runs it produces a lot of carbon monoxide, and some nitrogen dioxide and also passes through some traces of unburnt vaporized fuel. Carbon Monoxide and unburnt fuel are bad for the environment, and NO2 is bad for people. A catalytic process is a chemical process that changes a substance. The catalytic converter turns the unburnt fuel vapor and carbon monoxide into carbon dioxide, a much less harmful substance, and water (this is why water drips from the tailpipes of gas engined cars). It breaks the nitrogen dioxide or NO2 into simple nitrogen. Mandating use of catalytic Converters lowered pollution emissions that cause smog, especially in dense urban environments, though some people feel that modern cars are efficient enough they no longer need the expensive heavy part.