Some writers choose to write from multiple points of view. There are reasons to do this if say a personal perspective is needed along with a third person overview to describe events at which the first person narrator is/was not present.
In Edith Wharton’s ETHAN FROME, a first person narrator speaks in a prologue and epilogue while a third person voice is used to tell the story in the middle. The narrator, whose name we never learn, moves to Starkfield, New England, where he meets the eponymous Frome and at the end of the book, is forced to spend an evening at Frome’s house where he bears witness to the effects of a tragedy 24 years earlier.
The third person voice then tells the story of that long ago tragedy, of the adulterous love between Frome and his wife’s cousin Mattie, and of their pact to die together in a sledding accident. It is left to the first person narrator to discover firsthand the dreadful effects that accident.
The catch here is that the unnamed narrator is also telling the third person story which he says he has pieced together, apparently from gossip.
The reader is meant to believe him, we think, because he offers the only link between the bleak world of Frome and that of the reader. Also, Wharton offers no other view of the story. But the narrator-disguised-as-a-third-person cannot possibly have gleaned the whole, true story of the closely guarded accident from the townspeople and what he has garnered is slanted towards Frome’s point of view. Who knows what Mattie and Frome’s wife think? The limits on the narrator undercut the believability of the story.
These are the kinds of considerations to be made when considering a multiple viewpoint. Other first/third examples include TREASURE ISLAND and BLEAK HOUSE. Tim O’Brien makes an effective switch between third person omniscient and third person limited to one character’s point of view in the title chapter/story of THE THINGS THEY CARRIED. He uses the omni point of view when writing about Alpha Company and Lieutenant Jimmy Cross’s viewpoint when writing about his love for Martha. It is beautifully specific.