You can use the glossy varnish as an airbrush to strengthen the adhesion of the paint: in fact, the glossy varnish will allow you to create a sealing layer before painting. This technique is very practical on reversed leather, suede, nubuck, etc.
When applied as a finishing layer, glossy varnish provides shiny reflections.
For example, you can apply it to the eyes of a character to create a realistic reflection.