There are many reasons why authors choose to tell stories in the first-person. One of the most important reason is the sense of immediacy and intimacy that first-person narration provides. The reader is very quickly drawn into the narrator’s world and a relationship is immediately formed. Your question specified F. Scott Fitzgerald’s The Great Gatsby and Kurt Vonnegut’s Cat’s Cradle as two prominent examples of stories told in the first-person. Literature has been written in this manner for thousands of years. The Story of Sinuhe, for example, was written in Ancient Egypt, as far back as the 20th century B.C.E., and includes the following introductory passage:
“I was an attendant who attended his lord, a servant of the royal harem, waiting on the Princess, the highly praised Royal Wife of King Sesostris in Khenemsut, the daughter of King Amenemhet in Kanefru, Nefru, the revered.”
During the 19th century, famed American writer Mark Twain (Samuel Clemens) wrote The Adventures of Huckleberry Finn using first-person narrative:
“You don’t know about me, without you have read a book by the name of The Adventures of Tom Sawyer; but that ain’t no matter. That book was made by Mr. Mark Twain, and he told the truth, mainly.”
In these and most other instances, the author chose to convey his or her story through the perspective of the main protagonist, using first-person narration to connect the reader to the story. Third-person narration can similarly be used to convey the perspective of a protagonist, as when an omniscient narrator provides the reader on ongoing commentary concerning the protagonist’s actions and thoughts. Sometimes, authors simply choose to tell their story directly through the eyes of a narrator, even when that narrator is not the central figure.
In the case of Fitzgerald’s The Great Gatsby, Nick Carraway plays a major role in the story, and facilitates the actions of other characters, but the novel is titled The Great Gatsby for a reason: It is about the character of Jay Gatsby as seen through the eyes of the novel’s narrator. Let us look at Nick’s first mention of Gatsby:
“When I came back from the East last autumn I felt that I wanted the world to be in uniform and at a sort of moral attention forever; I wanted no more riotous excursions with privileged glimpses into the human heart. Only Gatsby, the man who gives his name to this book, was exempt from my reaction—Gatsby who represented everything for which I have an unaffected scorn.”
In this passage, Fitzgerald reveals that his narrator and protagonist is recording his thoughts on paper for a yet-to-be-published volume. He has returned to the home of his youth in the American Midwest following a tumultuous period in New York City, where he had gone to work as a bond trader. His tenure in New York, especially on the insulated section of that gigantic city known as Long Island, was marked by his acquaintance with Jay Gatsby, as well as with his cousin Daisy, her distant, philandering husband Tom, and Jordan Baker, the woman who will enlighten Nick while representing the worst of the pretentious, ostentatious culture of the “old money” world of East Egg. Fitzgerald’s novel will revolve around the myth and reality of Gatsby, but he chose to tell his story about this peculiar figure from Nick’s perspective, using first-person narration to draw the reader in and give the novel a more intimate feeling.
Now, let us look at Kurt Vonnegut’s Cat’s Cradle, published in 1963. Whether Vonnegut’s story constitutes science fiction or fantasy—the former having a foundation, as the name suggests, in scientific fact—is immaterial. What is important is that Vonnegut, like Fitzgerald, chose to tell his exceedingly original tale through the prism of its main character, John, or Jonah (“Call me Jonah. My parents did, or nearly did. They called me John”). Also, Vonnegut, like Fitzgerald, has his narrator and protagonist inform the reader that he is writing a book, having previously abandoned one such effort:
“When I was a younger man—two wives ago, 250,000 cigarettes ago, 3,000 quarts of booze ago. When I was a much younger man, I began to collect material for a book to be called The Day the World Ended. The book was to be factual. The book was to be an account of what important Americans had done on the day when the first atomic bomb was dropped on Hiroshima, Japan. It was to be a Christian book. I was a Christian then. I am a Bokononist now.”
. . .
“Be that as it may, I intend in this book to include as many members of my karass as possible, and I mean to examine all strong hints as to what on Earth we, collectively, have been up to. I do not intend that this book be a tract on behalf of Bokononism.”
With these introductory passages, Vonnegut introduces us to his narrator and provides the latter a sense of humanity and disillusionment. Vonnegut’s personal history, however, imbues a very different atmosphere into his narratives than that evident in Fitzgerald’s work. A prisoner of war held by the Germans during World War II, Vonnegut witnessed the immediate aftermath of the firebombing of Dresden, the episode that would inform his later book, Slaughterhouse-Five. Readers of this latter book are fully cognizant of the visceral impact the author’s experiences during the war had on his literature. Slaughterhouse-Five, in fact, mixes the author’s first-person narrative describing the origins of that book and his military service while interweaving the fictional account of Billy Pilgrim. If Slaughterhouse-Five interweaves first-person nonfiction narrative with fictional science fiction, Cat’s Cradle is, from start to finish, a surreal depiction of science gone horribly wrong. What those two novels have in common is a nihilistic overtone that is enhanced by the use of first-person narration.
Another reason authors use first-person points of view or perspectives is to deliberately confuse the reader as to what is reality and what is fiction or fantasy. Tim O’Brien’s The Things They Carried uses first-person narration, and the narrator’s perspective, to convey the sense of tragedy experienced by the author during his tour of duty in Vietnam. The point of O’Brien’s volume, however, is to confuse the reader as to which details and stories are true and which are fabrications. The Things They Carried is nonfiction fiction, the distinction neatly blurred to protect the innocent/guilty. Late in his book, O’Brien is describing the experience of killing another human being. He needs to cleanse his soul, but he doesn’t really want to admit to such an act. He understands that famous line from the old Western film The Man Who Shot Liberty Valance: "When the legend becomes fact, print the legend." Here’s O’Brien’s interpretation: “I want you to feel what I felt. I want you to know why story-truth is truer sometimes than happening-truth.” O’Brien uses first-person narration because it is his story. Depicting the atmosphere in Vietnam during that politically-divisive and bloody conflict through the eyes of someone who was there provides the narrative a greater immediacy and a much greater emotional weight.
Fitzgerald and Vonnegut employed first-person narration because it provided the perspective and immediacy they hoped to convey. Told through the eyes of the protagonist in his or her own voice creates a connection to the reader that is more personal than through third-person narratives.