This plugin is for multi-language sites where most of the content is translated, but some of the content is only available in a single language - but multi-language menus cause duplication of pages, resulting in duplicate content.
For example, if you had a site with English and Spanish, but there was a blog that was available only in English but you had English and Spanish menus pointing to the same category, then you might end up with something like this:
ttp://www.example.com/en/blog/
ttp://www.example.com/es/blog/
This plugin would add a