Tadashi Yamaguchi's development as a pinch server in the anime "Haikyuu!!" is significant because it showcases his growth, determination, and dedication to improving his skills despite initially being a benchwarmer. His journey as a pinch server highlights the importance of perseverance, hard work, and seizing opportunities to contribute to the team's success, even in a challenging role. It also emphasizes the value of being reliable and making a difference in the game, no matter the position one plays on the team.