I found very great answer from John Mueller that he posted about Site Map in Stackexchange. .
A Sitemap file helps search engines to discover new and updated URLs on your website. In particular, if your website is fairly large, then this can help them to be able to focus on the new & updated content, instead of having to blindly crawl through everything to see if anything has changed. That can result in new content being found much faster, which can be quite noticeable especially if the site is larger or more complex.
Additionally, you can use several extensions in Sitemaps files (eg for images, video, News, or internationalization), should you choose to do that. These extensions are all optional.
For most websites, the most visible element of Sitemaps files is that you can see the indexed URL count. It can take a day or so to appear, so if you just submitted a Sitemap for the first time, you may need to be a bit patient. While other ways (eg a site:-query) are very, very rough approximations, this count is extremely acccurate.