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You should use it when your documents containing a much of contain many low-level headings and you don't want to edit all of them separately.
For example on this sandbox only Heading 1 - Heading 4 was were made editable. Heading of this part is 4th-level and heading above below is 5th-level. So when you click EDIT above you will be able to edit two parts of text together
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Pay your attention: there is no EDIT-link near heading above.