The rivalry between Karasuno and Inarizaki in the story of "Haikyuu!!" serves as a major plot point that drives character development, intensifies matches, and adds excitement to the overall narrative. It pushes both teams to strive for improvement, showcases the strengths and weaknesses of each team, and creates intense and memorable moments during their matches. Additionally, the rivalry highlights themes of perseverance, teamwork, and sportsmanship throughout the series.