There are two kinds of Site Maps. One is designed for humans, and the other is for search bots like Google, Bing, Yahoo, and Ask.
Human Site Maps
The navigation menu, which usually appears on the left of the page in larger layouts, or at the top and bottom of the page in mobile layouts, is usually collapsed to show all the major topics, and the minor topics within the current major topic. This usually works fine, but it is possible for a viewer to visit your page without realizing how much information is actually there. The site map gives the viewer a chance to see pages they might otherwise have missed. For an example, look at JWebs.bz’s Site Map. We will automatically create a Site Map page for you if your site contains major and minor topics.
Search Bot Site Maps
Another kind of Site Map is one that is designed for the robots that continually browse the web to see what pages are new or changed. When a page on one of our websites changes, it also changes this Site Map. So the next search bot to browse the site looks to see if it has this kind of Site Map. If it does, Google, Bing, Yahoo, and Ask (and perhaps others) will use this information to help it figure out which pages to search, and how often to search them. All of our websites have this kind of site map. A small change on a page, even a new news item or calendar item, will update the date on the site map.
If you want to see what the search bot sees, you can look at it HERE.