Sitemaps help visitors find what they are looking for.

Now not all of these visitors are people. Search engines like Google send robots to crawl your site and index it.  The index is the information they use to give good results when people use their search service.

Since most websites get a huge number of new visitors from search engines like Google ..... you might want to help them out and make it easy for them.  A sitemap does this. 

This should be the first SEO option you put on your website.