Because this protocol allows a webmaster to inform the search engines about URLs on a website that are available for crawling.
Some websites doesn't need this. But sitemaps are particularly beneficial on websites where:
- some areas of the website are not available through the browsable interface
webmasters use rich Ajax, Silverlight, or Flash content that is not normally processed by search engines.
- The website is very large and there is a chance for the web crawlers to overlook some of the new or recently updated content.
- When websites have a huge amount of pages that are isolated or not well linked together, or
When a website hasn't enough external links.
Learn more:
https://en.wikipedia.org/wiki/Sitemaps.