It's for UV protection and appearance. If you never worked with epoxy or varnish, you'll find out thats it's next to impossible to get an even final coat of epoxy without a few runs that you have to sand. You're also dealing with pot life and a thicker viscosity.

The varnish is easy to brush on a thin coat and get a nice final gloss on your boat. Epoxy will eventually break down in the sun. It gets weaker, can cloud and will almost disipate into the air. (like evaporation) without a UV protectant.

You sand a lot off between coats even with a extremely fine sanding - so build up is really never a problem. As far as abraision protection, it gives you none. Matter of fact it scuffs easily.

Nick's brother Eric uses white cedar below the water line. A smart move. It shows scuffs and scratches less.