Varnish makes things shinier, yes, but it doesn’t make things smooth. What varnish does do is remove the duskiness of dark colors and enhances the sparkle of metallic and pearl polymer clay.
Think of it like wetting a beach pebble. Or putting lotion on dry, winter skin.
Sometimes varnish is the right finish for polymer clay, sometimes it’s not. It just depends on the effect you want to create. Knowing your materials in one of the most valuable things you will learn when mastering the medium of polymer clay.