Detailing clay bars when combined with a quality detailing spray easily remove contamination from the bodywork and wheels. Imagine a film of dirt over a mirror and how that would affect the reflection. Well that is exactly what contamination does to your paintwork diminishing it's finish. The clay bar and spray remove the contamination and restore a smooth clear finish. If you are looking to polish your paint then using a clay bar is essential.
Quick detailing sprays are not only for use with clay they have another benefit too as they help seal the paintwork and introduce amazing levels of gloss when used with a microfibre cloth.