The summary says it. Use case - packages that can be fetched automatically, but license doesn't allow mirroring and upstream changes the distfile keeping the same URL. This would allow to display an explanation to the users so they don't keep retrying or redigesting own file or etc.
Example: sci-geosciences/googleearth see e.g. bug 336331.
This is fixed in git:
This is fixed in 184.108.40.206 and 2.2_rc79.