Oil, grease and brake dust cling to everything: wheels, brake calipers, undercarriages, engine blocks. Regular car shampoo leaves that layer right where it is. A good degreaser chemically breaks down the fat molecules so they rinse away, and preps the surface for the next step.

Degreasers are also essential for paint prep before coating or wax. A trace of polish, silicones or fingerprints under a coating leads to poor bonding. Always wipe the paint surface with a degreaser after polishing, then you know you're starting with a truly clean canvas.