The principle
The project is the working container. The deliverable can be smaller.
A book project can also contain articles
An archive folder may hold published essays. A book part may need to go to an editor before the rest is ready. A proposal may require only sample chapters. Exporting everything would be the wrong unit.
Hierarchy already defines useful boundaries
If a section contains subsections, exporting that branch lets the existing structure define the package. You do not need to copy the material into a temporary project simply to produce a smaller file.
This is especially useful when articles become a book
The same project can preserve old published pieces while producing one current article, one book part or the complete manuscript as separate outputs.
Export should not determine project structure
Organize the project for writing and thinking. Choose the export boundary when you know what the recipient actually needs.