F. Scott Fitzgerald was undoubtedly a genius and wrote beautiful prose, and the use of Nick Carraway as the narrator is reminiscent of the way Hollywood employs music in soundtracks to make the audience feel the way they are supposed to feel about what iss happening on the screen.
Nick not only tells the readers what has happened but continually suggests how they should feel about what is happening. He does this mainly by expressing how he feels himself. This seems to be Fitzgerald's main reason for using Nick as a minor-character narrator.
If Fitzgerald had written the novel as a third-person anonymous narrator, he would have found it difficult or impossible to tell his readers how they should be feeling; and it seems doubtful that he would have been able to evoke the same feelings that Nick expresses as narrator. We might not feel any sympathy at all for Gatsby without Nick's sycophantic input. Nick is like a cheerleader pumping up the crowd. When Gatsby explains to Tom that he went to Oxford for five months after the war, Nick comments:
I wanted to get up and slap him on the back. I had one of those renewals of complete faith in him that I'd experienced before.
Without Nick rooting for Gatsby all the way through the book, the reader might form a different opinion. In truth, Gatsby is a mobster who uses his illegal wealth to break into the upper class. He uses people, including Nick, throughout the book. Gatsby may even be using Daisy as a means of achieving upward social mobility. He is trying to break up Tom and Daisy's family, however toxic it may be. Nick's narration helps the reader put these less attractive qualities into context and sympathize with Gatsby, despite his faults.