When reading a novel, we accept that it was written by the author named on the cover and title page. While willingly suspending disbelief, we imagine simply that the fictitious events are occurring, not that the author or anyone else is narrating them to us unless one character narrates in the first person or a narrator directly addresses the reader with comments like "You must understand that..." etc.
When a story is told from a single point of view, whether first or third person, then the narrative is limited to what the view point character could have known whereas the impersonal narrator who is not a character yet addresses the reader shares the author's omniscience about the characters and their environment. However, this narrator is not the author. The author when writing the novel creates the characters whereas, when suspending disbelief, we accept that the narrator is informing us about characters who would have existed even if we had not been informed about them. The omniscient narrator is part of the fictional process although not one of the fictitious characters. He is intermediate between author and characters. By writing in a different style, the author would create a different narrator. A prose style in which the reader is not directly addressed has an invisible and virtually non-existent narrator.
If attention were focused on the omniscient narrator, then he would be seen to have a god-like relationship to the world inhabited by the fictitious characters. However, attention is usually focused on the content of the narrative, not on the process of narration. When a novelist cameos as the first person narrator of one section of a novel, then by implication this character narrates the entire text and cannot consistently share the author's omniscience even though s/he is a character based on the author. The analogy with the idea of divine incarnation is striking.
Some texts, e.g., Doyle's/Watson's Holmesian memoirs, are published both in our world and in the fictitious world that they describe. That explains why there is a first person narrator. He is informing not us of fictitious events but his contemporaries of, to them, real events. Allan Quartermain read She, ending in She's death, then sent Rider Haggard the prequel, She and Allan.