As far as I can tell this seems to be for deleting application data (~/.var/app/*), whereas flatpak uninstall --unused is for uninstalling runtimes that are no longer needed.

This app is more equivalent to flatpak uninstall --delete-data

Because there are distros, like fedora for one, that have flat packs installable by the likes of discovery on KDE that doesn’t require CLI useage for install or uninstall of flatpacks