Having these on hand allows you to create contrast effects within your paintings. Placing a gloss finish next to a matt finish can imitate effects of depth. Play with them and see.
These don’t tend to dry as slick as Varathane coats and they are a little more costly to use as diluting liquids for making hues and layers.
Warning: They also dry remarkably fast on the brushes, so keep some warm water nearby to rinse the brushes after each use.