Nick & Jordan

I have personally been wondering about the fact between Nick and Jordan and their relationship. Little is said of them, they both speak little and do not seem to play an important role in the heat of the action. They both act as witnesses to the madness; Nick recounts the story for the audience in his eyes. I'm curious as to why F. Scott Fitzgerald made Nick the narrator if he was not a major part in the theme and plot. Also, I wonder why neither of the two have elaborate backgrounds or characterization. Of course we are told a few things, but nothing near to what we know about Tom, Daisy, and Gatsby. I just find it interesting that the narrator of the story has little description and little role.