The site's header and footer — what repeats on every page.
Build once, seen everywhere
A region does not belong to a page. A change here affects every page of that site, from the home page to a blog post to a product page.
So before changing the header, be aware that the change cannot be tried out on one page.
Built with the same tool as a page
A region is edited in the same Studio and has the same draft and published versions. The header can be edited and saved without the site changing, until you publish it.
Regions are tied to the theme
Each theme has its own places for a header and footer. Changing the site's theme can move a region somewhere else — or leave it nowhere to sit.
Some elements only make sense in a region
Elements such as the light/dark switch are built for headers and footers, and do not appear in the element list of an ordinary page.