A sitemap is often considered redundant in the process of building a website, and that is indeed the fact if you made a sitemap for the sake of having one. By highlighting the importance of having a well constructed sitemap, you will be able to tailor your own sitemap to suit your own needs.
1) Navigation purposes
A sitemap literally acts as a map of your site. If your visitors browses your site and gets lost between the thousands of pages on your site, they can always refer to your sitemap to see where they are, and navigate through your pages with the utmost ease.