As other's have noted the documentation can make this module somewhat frustrating. I put most of this down to my being an American and the author being German. There are two terrif reasons to use this tool
1) Ability to have "split menus" This means that you can have one menu dependent on selection of another. For instance as uses select items from the horizontal menu at the top a left side menu can be configured to show the sub elements for that selection.
2)Drop down menu's without javascript (almost). This tool makes suckerfish CSS menus a snap. For more on drop down menus see:http://forum.joomla.org/index.php/topic,22023.0.html
A real time saver and worth some experimentation to figure out what it does.