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 you apply it afterwards, to varnish, you will obtain shiny reflections.
For example, you can apply it to the eyes of a character to create a realistic reflection.