I've been told that I need to have a site map on my website but I didn't know why.
So according to Wikipedia, here is the definition of a site map:
A site map (or sitemap) is a representation of the architecture of a web site. It can be either a document in any form used as a planning tool for web design, or a web page that lists the pages on a web site, typically organized in hierarchical fashion. This helps visitors and search engine bots find pages on the site. While some developers argue that site index is a more appropriately used term to relay page function, web visitors are used to seeing each term and generally associate both as one and the same. However, a site index is often used to mean an A-Z index that provides access to particular content, while a site map provides a general top-down view of the overall site contents.