It is a contractual requirement. Captains seat to do with as he pleases.
But do bear in mind, imagine yourself sitting at your desk with nothing to do for 13/14 hours or so… very boring ( on long haul flights that have a cruise pilot aboard the captain/FO may spend 6/7 hours either sleeping and or watching a movie, maybe reading a book or manual) so they are ready to be fresh and alert for the following critical phases of flight.
You want the pilot to be able to get out of the flight deck to completely detach and refresh. (how many have tried to just take a quick 10/15 minute break at your desk, its hard to do, your always thinking about work, checking papers on your desk, same for pilots)