sitemaps do not affect indexing or ranking - they are simply a way to tell search engines about pages on your site that they might not otherwise find.
XML sitemaps can help speed indexing. But it's no guarantee for getting all pages indexed and it's not a pagerank booster
Here is 2 type of site map first is sitemap.html and second is sitemap.xml ,i am discuss sitemap.xml this is very useful index all site url in google database but no guarantee for getting all pages indexed and not any role of improve page rank and traffic but sitemap.html improve your traffic because site given all services and information in sitemap.html page.