Small sections reduce cognitive load while writing. Large continuous reading reveals problems created between those sections.
Chapter editing hides transitions
A chapter can work perfectly in isolation and still arrive at the wrong moment. The weak spot may be the last paragraph of the previous chapter, a repeated explanation three chapters earlier, or a pacing problem visible only across a long stretch.
Exporting a temporary merged file creates another artifact
You can always export to DOCX or PDF just to read continuity, but now the reading copy and the working project diverge. A continuous manuscript view keeps the reading pass over the same underlying sections.
The full view is for rhythm, not necessarily drafting
Use it to inspect chapter balance, repetition, transitions, pacing, and the emotional or argumentative flow of the book. Switch back to a single section when the task becomes local again.
The design principle
Structure should make a large book manageable without forcing the author to experience it as fragments forever.