Most pages typically make use of the global top and bottom areas, but there are good reasons sometimes when this is not possible. For example, you may have a hero bloc on the home page or other pages with a change in layout. Sometimes they even include the same custom brics, yet moved outside those global areas.
On a large site with many pages it can be time consuming clicking through one page after another to see whether or not it makes use of the global top & bottom areas just to make a small adjustment across certain brics. We could really use some means to quickly identify pages that are not making use of global settings without having to check them individually one by one. When you come back to a site after 12 months, chances are you will not remember which pages did not use global areas.