Site map used to be a standard feature for all websites, usually a one-pager that gives users a clear overview of the website’s structure. Jakob Nielson, the self-proclaimed usability guru has said this about site maps,

Users go to site maps if they are lost, frustrated, or looking for specific details on a crowded site. A site map’s main benefit is to give users an overview of the site’s areas in a single glance by dedicating an entire page to a visualization of the information architecture. If designed well, this overview can include several levels of hierarchy, and yet not get so big that users lose their ability to grasp the map as a whole.
(posted in 2002)

I have never believed in site maps: if your site’s visitors need to use them, the website’s design has failed. The site map serves as a temporary relief to the bigger problem of poor overall design, that the users would eventually find a certain page or section via the map. That could have the negative effect of them not complaining about the poor design, where as if you don’t have a site map, the frustrated user will contact you - and you’ll have to fix that problem right away.