I am not the author of the plugin but what I understand is that it's a plugin that autogenerates the sitemap.xml. This file has metadata that are used by search engines. So, having a sitemap.xml would probably help the search engines to find your site.
However, Lukas said that out of the box, Pier sites are perfectly indexed by Google. Recent versions for example define the canonical URL for all pages (
http://tinyurl.com/b9saag).
To use it, just add "sitemap" somewhere (at least this is what I did).
Cheers,
Mariano