Search engines like Google and Bing want us to submit sitemap file in webmaster tools so that they can use it to crawl and analyze our website. If you still have pure HTML based website during 90's and early 00's, most likely you have to run sitemap generator software from PC, crawling and collecting urls from your web pages and save into a sitemap file. The major problem is you need to generate new sitemap file every now and then when you update your website, then upload to webmaster tools.
With Joomla, you can use Xmap, a free component created by Guillermo Vargas since Joomla 1.x, to re-generate dynamic sitemap automatically so that you can totally forget about sitemap file after initial setup. Xmap creates sitemap based on the structure of menus in your Joomla website. You can add or remove menus anytime and Xmap will dynamically generate sitemap accordingly with additional metadata. You can also create any number of sitemaps with different options. However, Xmap also comes with poor documentation and poor support, as you can see from JED comments. If you encounter any problems, you need to search the solution from forums, just like Xmap was not functional in recent Joomla 3.2 upgrade.