If your website supports multiple languages then this module is extremely important. It will help you increase traffic from countries with languages that you support.
Specifically, as you know, google search engine supports many countries with different languages, for example google.de google.co.jp, google.es, ... If your website supports many languages, but you don't tell google about this, then your search results will not appear on other google sites (for example google.de).
Zen Cart Sitemap for multiple languages will help you on this, it will help Google to understand what your website is supported, displays search results with different languages on the google domain (.com, .de, .es, ...). So your website will increase the traffic from these countries.
You can read this article for more information:
To use this service, your site must support the Simple SEO URLs plugin.