There are two types of site map. HTML sitemap and XML sitemap.
HTML Sitemap is for website visitors so that they can find all pages on that website if the site is large.
XML Sitemap is for search engine spiders for better indexing of web pages.
XML sitemap is more important for website indexing.
In my opinion: very low importance.
I had websites and blogs with basic sitemaps and they did the same like others with very consistent sitemaps. There are a lot others factors more important for SEO.
how can i submit XML sitemap for website indexing? I made some changes in website but still google did not crawl my website. Do i have to submite XML sitemap.
Sitemaps make navigating your site easier and having an updated sitemap on your site is good both for your users and for search engines. Sitemaps are an important way of communication with search engines. While in robots.txt you tell search engines which parts of your site to exclude from indexing, in your site map you tell search engines where you'd like them to go.