Coating refers to the application of a thin vinyl film on the surface of the printed substrate for the following reasons

To protect the surface and prevent tearing.
Glossy coating provide a shiny surface, while matte coating provides a calm effect.
Applying matte coating onto the printed substrate before applying Scodix effects will make Scodix more visible with a slight shiny effect.

Coating is also used as a method to bond the film. It is well suited to be used onto Snow or Art paper with smooth surface while paper with different texture such as Synthetic or Rendezvous create a more dimensional coating effect.
Coating will not reduce the paper quality, however, paper with basis weight of 100g or less is difficult to coat compared to paper weight 100g and above.