Chapters are not necessary, but help readers understand what is happening.
There ARE chapters in films, signaled by "establishing shots", the first orientation shot that tells the viewer the time/place has changed and a new scene will take place there. Typically the image fades out and fades in on a distinctly different scene, often (not always) a long shot of a building, city, planet, etc, before cutting to some closer setting (in a room, on a campus, etc). For examples of such a change:
- from night and Alice opening her front door to find Bob, to a morning alarm clock (establish time and place) then a pull back to show Alice and Bob tangled naked in bed,
- from Alice sitting in her office (during the day) angry, to cautiously exploring a darkened parking garage,
- from the FBI briefing room to an open rural field in bright sunlight with everybody in assault gear, on stealth approach to a farmhouse.
Listen very closely to the soundtrack of the film a few seconds before these scene changes: You will (frequently) hear the soundtrack of the NEXT scene (or something in it, like an engine or teakettle) start to fade in while in the current scene dialogue has been completed and the scene wraps up. That is the done intentionally to prepare the
reader viewer for a transition (and has something to do with physiological perception and how long it takes to recognize audio clues vs visual clues, and is something directors and sound engineers do, not writers).
These transitions cut out a big chunk of boring time (or explicit sexuality or violence not suited for the target audience). We don't need to see the FBI agents wrap up their meeting, go to the restroom, change into their gear, pile into the SUV and eat Fritos and debate whether Star Trek is "politically realistic" for an hour while they ride off to this remote farmhouse.
Chapter breaks in Novels serve a very similar purpose, they prevent confusion in readers. Like some movies, the chapter breaks when the POV character changes. In a Romantic Comedy (say You've Got Mail or Sleepless in Seattle or When Harry Met Sally), the movie switches between the male protagonist and the female protagonist (there is often no explicit antagonist working against them, other than the environment and life circumstances that separate our lovers-to-be).
Alternatively, with a single POV in a novel, the chapter changes for several reasons; the most common being:
- a passage of time and/or place (or setting; eg outdoors to indoors, a classroom on Earth to the bridge of a starship, USA to France, etc).
- to skip over boring investigations or preparations begun in the previous chapter,
- A discrete "fade to black" because the author finds explicit sex scenes cheesy,
- To change the tempo or style of writing; e.g. to exposition about the long passage of time, or a trip, that the author does not want to excise completely with a jump but also does not want to portray in "real time" with dialogue and POV character thoughts. So the author can describe the trip and its hardships. By "trip" I mean even figurative trips, for example an eight month hospital stay with multiple surgeries. The author could say "eight months and three surgeries later," or, "Josh lay in a hospital bed for two months and endured six more of painful rehab and trauma therapy." But that experience surely changes Josh and those transitions do not capture or explain the "new Josh" that wheels himself out of the hospital eight months later.
I am sure there are many other good reasons to break to a new chapter, these are off the top of my head.
They exist for much the same reason establishing shots and scene breaks exist in film; we don't watch the entire trip of Luke getting to Yoda, it just so happens that whenever Luke has to do something boring, Hans Solo is doing something interesting, and vice versa! A wonderful co-inki-dink.
We write chapter breaks for reader clarity. They can be one sentence long (there are famous examples), or very long. They help orient readers quickly and accurately, put them in the right frame of mind to mentally wrap up the previous chapter as "done" and start a new scene with expectations of needing a re-orientation of where and when the story is, and which characters are involved.