No, its not necessary to have one but its a good practice to generate one for your site. An XML sitemap have a list of all pages in your website with some important information like their priority in your website, their changing frequency and others. When you submit this XML sitemap to search engines, it helps in better indexing for your website.
if you have proper navigation, then you don't even need a sitemap. all depends on how good your navigation is and how easy it is for googlebot to crawl through your existing sitemap
We create XML site map to search engine point of view, When you submit this XML sitemap to search engines, it helps in better indexing for your website.
Well XML Sitemap is Not Necessary for a Website but it can be Fruitful for a webmaster to submit XML sitemap for indexing web pages with Ease..
If you are looking for the Good SERP & PR .xml sitemap is very much essential. Because SE Robots can easily crawl & index through the .xml file
It is always necessary to have an XML site map in your site as there you put all the live URL's of your web site and search engine robots crawl those URLs on a regular basis to check any new update. XML sitemap helps in telling search engine robots about the new pages and update in pages, and robots crawl all the pages and index them in search engine data base.