A rescue.
In the story. Luffy saved in one form or another, ALL the strawhats from an oppressor:
Zoro from Helmeppo
Nami from Arlong
Usopp from Kuro
Sanji from Morgan
Chopper from Walpol
Robin from the CP9
Franky from the CP9
Brook from Moria
Jinbe from Hody
This rescue is important because it is for that reason that the strawhat mostly sees Luffy as a reliable captain.