I've never noticed an impact in search rankings, but it helps search engines keep up to date with your content. For instance <changefreq> will alert search engines to crawl your pages for updates at intervals; as chauhan said, a sitemap will also help every page on your site get indexed. In my experience <priority> hardly makes a difference.
I am agree with "chauhan". Sitemap provides a path to spider to craw all links available on websites. Sometimes spider not able to crawl inner pages of website but if you have create xml file then it will help it to reach on inner links easily.
Website sitemap are two types One is HTML sitemap and second one is XML sitemap. now we can explain here only XML sitemap.
XML sitemap are created in website only for search engine point of view because sitemap is a table of content of all the web pages that way when crawler move in your website then your all the webpages are going to index by Search engine crawler and for this your website domain value are going up in search engine eyes.
Creation of an XML sitemap won't improve your website's Google rankings directly, it rather makes your communication as a webmaster with Google easier.
An XML sitemap is an accepted format that Google comprehends best (contrary to HTML). It's a file that lists all the pages you want to show up in SERPs with info on how frequently you actually update them. BTW, there are tools that make the job easier, e.g. WebSite Auditor which comes with a built-in XML Sitemap Generator