Thefirst benefit of having Nick narrate the story is giving us a more realistic story. His point of view, that of a relatively objective observer, allows us to see the story through very accurate eyes. Through Fitzgerald;s stroke of genius, the novel draws us into itself, and the I that Nick uses to relate to himself becomes the I or;eyes; that the reader uses to see the story.
A part of this, Nick;s believability, lets the reader identify or sympathize with his values. Gatsby;s wealth or the riches of Tom and Daisy do not easily sway Nick. He is a tolerant person, but not overly so. An example of this is seen as Nick puts up with Tom;s racism and bigotry, but in the end he tells Gatsby that he is ;worth the whole; dang ;bunch put together;. He is very honest, but not Puritanical or narrow-minded. The objective viewpoint from which Nick tells us the story adds a sense of realism to The Great Gatsby. Without Nick, the other characters would be loud and flashy, not the eccentrics that they are.
Secondly, Nick adds information to the story through quite a few avenues. His position in the story with each of the characters makes him a key player in it. He moves in to the house next door to Gatsby. He is also Daisy;s cousin, and coincidentally was in the same senior society as Tom Buchanan at Yale. Nick meets Jordan Baker at the Buchanan;s house, and they become friends ( it is symbolism in the great gatsby), although we later see the beginnings of a romance blooming.