What the clay bar does it removes the surface contaminants, like paint over spray, tree sap, bugs, fallout, etc. from the car's surface. The best way to test, is to put your hand inside a small sandwich bag and place it on one of the top surfaces of the car i.e.( the hood, roof, fenders, or trunk) and gently, with light pressure rub across the surface. Can you feel any bumps or roughness? Clay it! If not, skip it. The sandwich bag test is important because our hands are too soft to actually feel the contaminates in the paint.
The main goal of the clay is to remove dirt and surface contaminants before you polish and wax.
Hope this helps
Brad