Advantages of Varnish
Provides radiant beauty to the wood.
Helps in protecting the wood by binding the surface.
Helps to prevent hairline cracks and absorb the effects of scratching and external damage.
Helps in reducing the transfer of moisture between the wood and the surrounding weather.
Provides softness and lasting flexibility.
Correspondingly, why do you need to varnish wood?
For instance: Spar varnish is typically used on wood outside where protection from UV light and water is critical. Polyurethane is often used for wood floors because it forms a very hard, tough surface that protects against abrasion.