Nick's role in the novel is both the narrator and commentary on the events that he is relating. Nick's views of morality are the criteria by which we judge the other characters. As an outsider to the social world depicted in the novel, Nick is able to observe and comment on the behavior of all other characters in the novel. Because of this, Nick can largely be seen as an objective narrator. Nick himself even says that he was "included to reserve all judgments," leading the reader to assume that what Nick is reporting is what actual happened in a reliable way.
Nick is also the medium used by Gatsby to get closer to Daisy. It is under the guise of visiting Nick that Gatsby arranges to see Daisy for the first time, and through Nick that Gatsby continues to rekindle his relationship with Daisy.