What did F. Scott Fitzgerald achieve by using Nick's point of view to tell Gatsby's story ?

Nick carraway is made the narrator he is a reliable one he tells us that he reserves all his judgments therefore the reader is lead to believe what he is saying is coming from an 3rd person objective point of view. Nick does not come from "old money" or the filthy rich. Nick's history seems to be upper middle class and quite "normal" as opposed to the vapid elite of East Egg. Nick is a pretty credible character. He doesn't have any eccentricities and seems grounded. He was educated at the Ivy School Yale and fought in WW1. Nick isn't a character who needs anything. He seems to have enough money to settle down in West Egg. He has come to New York for work and to learn the bond business; he doesn't seem to be in a hurry to get started. For part of the novel Nick is more or less a flat character. We can see life in West Egg through him. Nick observes situations without giving judgment. This allows the reader to trust Nick as a narrator and finally a character in the novel. It is perhaps ironic that as Nick becomes more entrenched into the lives of Gatsby and Tom, he becomes less of a reliable character: just reliable enough to finish telling the story.