Google spiders always crawl the sitemap of the website to get each and every information in one place. Sitemap provides each URL in one page. So, yes it is necessary to keep sitemap file in our website.
A sitemap is a way for visitors and Google to know how pages connect to other pages within your site. For example, at the top of THIS page, you can see the following site map: "Internet Marketing > Search Engine Optimization". So now you know where you are within these forums and what level is the next level up. Some sites go deeper than that; they can have MANY more levels. And by allowing visitors to click on any of those links, they can quickly navigate to other locations within a site.
Site map is the map of any website, you can go directly any page using sitemap, it consist the structure os your website. In SEO it is very important as search engines incexed web pages according to sitemap
The sitemap is a very important and necessary component in each website. In short, the sitemap is an index page of your website, which offers links to all pages of your website. These links are categorized and organized in hierarchical form, which makes finding them very easy. In case the visitor of your website cannot find the needed page, he or she should refer to the sitemap.