This addon allows virtual folders (saved searches) to automatically process all subfolders of their selected target folders.
The search targets are updated automatically whenever a new folder is added, or a virtual folder is updated. This does not apply to the Inbox folder, as it contains all other folders, including folders that are usually irrelevant for saved searches (Trash, Junk, etc...).
This is especially useful when using saved searches along with the archiving feature.
See also the official addon's page