They show the overall structure and hierarchy of a Web Site without going into details of each page.
Combining a detailed site map and wireframes create a document that can guide client and developer in discussing the project, and then will help to instruct designers and programmers.
While it is possible to start web application development project without site map, it would increase the risk of costly future additions on the way to the finish.
Sample of the site map:
Another site map example: Social Network My Account Functions Site Map







