Varnish is essential for the protection of oil paintings. It provides a clear protective layer over the paint and prevents it from coming into contact with dust, moisture, and other environmental factors that could cause damage over time. Varnish also enhances the appearance of the painting by providing a uniform finish and increasing the depth and intensity of colors. However, varnish is not typically used on watercolor, pastel or gouache paintings because they are usually framed under glass for protection.