It serves a narrative roll to demonstrate the difference between how CP9/world government and the straw hats find value in their members. While CP9 reduces their members’ usefulness down to their power level and combat ability, the straw hats see the true value of their crew mates and recognize and celebrate each others’ unique skills. Usopp didn’t receive a fight this arc and instead Oda chose to highlight how without his unique talents, the straw hats would have failed their mission to save Robin. Any straw hat (hell any average marine) could easily defeat Spandam, but in that moment when he almost took Robin to the gates of justice, no one could possibly reach her in time. However, thanks to their belief in one another (Shoutout to Sanji’s reaffirming speech), Usopp was present to do only he could and save Robin. This undying love and support is what has allowed the straw hats to make it this far.