Fitzgerald created a character like Nick to narrate his the story of Jay Gatsby, Daisy and Tom Buchanan, and the other characters of "The Great Gatsby," because Nick had objective observation skills. As an outsider moving in, Nick was more beliveable. The story is told from Nick's perspective and thus allows the reader to see all perspectives of the world in which he finds himself. We see the traditional old money values and hubris of Tom and Daisy. We can see the carefree and idealic attitude of Jay. We can see Jordan's dishonesty and her lack of respect for the middle class values. We can see the suffering of George, who believes he is losing his wife, and the blindness of both George and Myrtle; both believing they will soon escape from the lonely lives they live. Through the use of a "good ole' guy" and a "good sport" like Nick we can trust his view. We, as readers like Nick and we believe him when he tells us that "Jay, you are worth the lot of them."
If Fitzgerald had used any other character, we would not have the whole story. We would have only seen one side of the story. We would have seen the the story from Tom's skeptical and mean side. We might have seen the half truths from Jay, Daisy, or any of the other characters. Only from Nick Caraway can we learn the whole story.
"Superbly chosen by the author, Nick is a romantic, moralist, and judge who gives the reader retrospective flashbacks that fill us in on the life of Gatsby and then flash forward to foreshadow his tragedy. Nick must be the kind of person whom others trust. Nick undergoes a transformation himself because of his observations about experiences surrounding the mysterious figure of Jay Gatsby."