It adds to the story tremendously. By showcasing the Celestial Dragons hunting humans for sport, it really hammers home the fact that they view everyone who isn't a Celestial Dragon as an insignificant ant.
It's one thing for Oda to constantly have the Celestial Dragons talk down on the human race, and another thing entirely to show just how little they perceive them as a race by having them actually treat them as game animals, and hunt them down in competition.