You have a multipart document, and you want to create headers and footers that go with each part. The picture shows examples of how you might want the headers and footers to differ to fit parts of the document.
For the cover page, you may want no page numbers at all, or any other header or footer text.
For front matter, such as the table of contents, headers and footers should reflect that this part is not yet the body of the document.
For chapters, headers can serve effectively as signposts to readers, reminding them of which chapter they're in and who wrote it.
You already know that the handy quality of headers and footers is that you can apply one once and have it appear everywhere. But for the multipart document, you need more control. How do you get that?