Feature summary
The intent is to have a user-configurable "fallback" page image if no other suitable image is available on a page.

Use case(s)
A template that is included on a group of pages includes an image that isn't a great option as a page image, but it's better than nothing.
Different groups of pages have a different "default" image suggested, so a site-wide configuration setting wouldn't be appropriate.

If the PageImages algorithm finds a decent image, excluding the forced scored image, that could be chosen.

For example: {{foo-nav}} contains a basic image of "foo" with a forced score of 1.
A page Rainbow foo may have a quality image that would override the default from {{foo-nav}}.
A page Plaid foo does not have any images, so would show the default from {{foo-nav}}.

Benefits
Allows more fine-grained control of the page image, without having to manually configure the page image on every page (as suggested by T91683).