I know submitting sitemap will help the bot crawl your site better, but will sitemap help to boost index and ranking for new site? I mean a new site that do not have any page index by search engine.
that is true. i normally tell people to add one at a bare minimum so they learn about inner linking of pages and how everything should flow from one page to the next within their site.It will only help your rankings if all of your pages are optimised well enough - because the sitemap tells the bots which pages to index and spider as stated above.
I'm getting mine updated to the latest version on my sites.Sitemaps are an easy way for webmasters to inform search engines about pages on their sites that are available for crawling. In its simplest form, a Sitemap is an XML file that lists URLs for a site along with additional metadata about each URL (when it was last updated, how often it usually changes, and how important it is, relative to other URLs in the site) so that search engines can more intelligently crawl the site.