Bathed in Sunlight 24X36"
Acrylic Painting (cropped)
Any one of the above benefits may be sufficient cause for applying an isolation coat.
With or without the isolation coat the painting will appear less chalky, dry and flat. The darks will look darker and richer. Some colours may come out at the higher and lower value ends.
Diminish or eliminate glossy and matte patchiness. For an acrylic painting this will depend on your pre-coating with an isolation coat(s). For oil painting, this will rely on how well the suface is sealed - the sum result of good priming, wet impimatura and oiling out.
Many of the mediums used in an acrylic isolation coat and at times, the acrylic paint itself, are sticky and can hold a static charge causing the surface to attract and retain airborne dust particles or smoke (from an increasingly burning world). However, a good varnish is a different matter, often having been modified not to collect dust or be sticky. Damar mixtures for oil painting varnish are sticky - no such benefit there.
A satin or matte varnish can reduce the glare produced by light shining upon a painted surface.
Protect the painting from mold, smoke, cooking grease and reactants or oxidants found in the air. Or the sticky fingers of small children from your white trash neighbours. In the case of a varnish containing UV barriers (inhibitors), from the ionizing effect of ultraviolet sunlight. A final top coat of varnish offers a surface which can be cleaned without damaging the painting.