The distinction
A chapter boundary is semantic. A page break is physical.
Not every section should start on a new page
A manuscript may contain nested sections that should flow continuously. Automatically treating every structural node as a page boundary would make the book fragmented.
Some boundaries must be deliberate
A new major part, an appendix, a title-like section or a special insert may need to start on a fresh page even if the hierarchy alone does not express that requirement.
Page breaks belong near preview
The useful workflow is visual: add a break where the reading experience needs it, then inspect the complete book preview. The break is not an abstract formatting command; it is a controlled transition in the final output.
Do not use page breaks to repair bad structure
If a section should actually be a chapter, make it a chapter. Page breaks solve layout, not manuscript organization.