The most likely purpose of Orwell's passage is to illustrate the power struggle that Napoleon will likely win.
This passage emphasizes the contrast between Snowball and Napoleon, highlighting Snowball's creativity and dedication to the windmill project, while also showcasing Napoleon's hostility and resistance to Snowball's ideas. The act of Napoleon urinating on the plans symbolizes not only his disdain for Snowball's efforts but also foreshadows his eventual dominance over the other animals and control of the farm.