Sitemap is an important element for search engines as well as for visitors. Suppose you have a large website with thousands of pages. Now, visitors will get confused about the way to navigate your site. A sitemap helps site visitors to navigate easily through your pages.
On the other hand, Search engine crawlers (robots) is able find your pages through your sitemap at one place. In this way the can crawl your all pages at a times. Hence, your pages become visible quickly at search results. Without any sitemap, indexing rate will be slow and your site will be less displayed on search results.