A Sitemap is your Guide, Finder, Help and listing of contents to help everyone find all pages within a website.
Every good website has one. A website will have many pages that are a branch of a "Main Page" that is listed within the Navigation Bar. Many of these pages are very useful and helpful to a reader.
However, they are difficult to find without a good "map".
Some very smart person thought of and designed a method to display all pages in a systematic method to assist everyone.