You need to understand how navigation works. It is the navigator that decides how many degrees the rudder needs to be rotated, at what speed, the ship needs to move, which path to take etc. etc. Helmsman follows the instruction of the navigator and steers the ship accordingly. Jinbei needs instruction from Nami and Nami needs the map from Robin.
In addition, Laugh Tale is likely not located in a calm area of the sea. If that was the case, people would find Laugh Tale just by happenstance (unless Laugh Tale is an unassuming island). More likely, Laugh Tale is located in a very difficult path that cannot be sailed easily like Skypeia. So, Nami will be useful there too. And before you bring up Roger Pirates going there, they also managed to go to Skypeia, so they definitely had a fairly good navigator (Scopper Gaban, iirc).