The global header and footer cover most pages, but not every page wants the same chrome. A landing page might need no navigation at all. A campaign page might need a different footer. Canvas handles this per page.
On any single page, you can choose to use the global header and footer, supply a custom header and footer just for that page, or show none. The choice is per page, so one override never affects the rest of the site.
This is what lets a clean landing page sit alongside your normal pages without compromise. The global pair stays the default, and you reach for an override only where a page actually needs one.