While pre-shading will give shade variations in the vehicle's image for an enhanced "3d effect", a wash is used to bring our the small details like panel lines, weld seams, depressions and creases. By adding a wash and defining the detail lines of the vehicles surfaces, it breaks the solid image of surfaces into details, thus giving it more life.
Also in some cases, a dry-brushed effect is necessary to add further to the 3D effect by highlighting the raised portions of the vehicle surfaces.