Imo it served two purpose :
-Excusing Garp relatively rushed defeat compared to Whitebeard as Oda couldn't dedicate too many chapters to Hachinosu due to Egghead being the main story
-Keeping Kuzan full arsenal for later, as Oda is an author who doesn't like characters showing their full power til it's their time in the spotlight. Or Kuzan is pretty much guaranteed to have an important role later.
-Excusing Garp relatively rushed defeat compared to Whitebeard as Oda couldn't dedicate too many chapters to Hachinosu due to Egghead being the main story
-Keeping Kuzan full arsenal for later, as Oda is an author who doesn't like characters showing their full power til it's their time in the spotlight. Or Kuzan is pretty much guaranteed to have an important role later.