The lifetime of a subdocment is associated with that of its parent. Subdocuments help to organize content within a document by externalizing it. This makes it easier for teams to work on a document together. Referencing parts of the document is also easier.
To improve the collaboration within a team, a larger document may be subdivided into subdocuments of type Section. The sections, as individual documents with a unique URL, makes it easier to be referenced from other documents.
The lifetime of subdocuments is associated with their parent documents. That is: if the parent document is removed, all subdocuments will also be removed. If a document stands for its own and should outlive the referencing document, it should be added as a root document to a doctype homepage.
The screencast shows how authors can organize their content in subdocuments easily.