The purpose of a coating is to protect the printed piece from dirt, smudges, fingerprints, scratching, etc. Coating also provides scuff resistance. And, yes, it can improve the visual appeal of the piece by providing a glossier and smoother finish. Protective coating provides surface protection for that postcard you did for your top account to arrive looking the way it looked when it was mailed. Printers who are aware that the postal service delivers a licking that keeps on ticking to printed pieces – and use that info to sell against unaware competitors – have an advantage.