An additional coating with 2-component clear lacquer is not absolutely necessary, as the last colouring layer is already a 2-component quality (with hardener), i.e. this layer is already fuel-resistant and high-gloss.
Usually the professional painter applies a 2-component high-gloss clear coat as a final protective layer.
The reason is because subsequent surface damage to the paintwork is much easier to repair than, for example, a paintwork repair directly on a not protected candy/pearl glaze. Polishing work is also easier on a 2component clearcoat because the colouring layer remains untouched.