I can make a good guess at the seat reason
Those of us with really nicely designed cockpits look at this and wonder why would you do this... Well because you don't have a really nicely designed cockpit.
This is a typical all out racer cockpit without back rests. If you have sailed for more than a race in one of these, you know that it is incredibly hard on the back. Uncomfortable fails to describe sitting on a moving ledge without any support for hours on end.
Since the boat does not look like a race only boat anymore, he elected to do something to make it so that the boat was usable.
A novel if somewhat ugly solution.. However solutions for the design problem of a very uncomfortable boat like this are rarely good looking, or functional. At least he went for functional and ugly instead of non functional and ugly.
Guy