Spar Varnish or Spar Urethane is a long oil varnish.
So it has a higher oil content then a pure polyurethane. This is partly why it takes longer to cure, and is techinically a softer finish then a pure poly finish.
The oil and the softness is a win in a bow since it leaves if flexible. If the finish is too hard then it would just crack. This is what happens with some of the new hardwood floor finishes they use in factories. They are UV cure finishes and are hard as nails, but don't take a hit very well. So if you drop something hard enough, the wood will dent and the finish will crack.
The finish is there to keep the wood from beind exposed to the elements and allow you to keep it clean, beyond that its all just about pretty.