To help users navigate your site. Sitemaps are very useful when interlinking important content on your site and hence providing a better experience for your website visitor.
To help search engine crawlers navigate your website
A Sitemap that:
Helps promote content themes, and thus communicate your site as an authority on a given topic. A “themed sitemap” is a great way to help users navigate your content, and let the engines know what your site is all about. Themed based sitemap will help create consistent interlinking by using similar keywords in anchor text thereby improving the inbound link quality of the pages that you are linking to.
Directs the user through a logical navigation map in an easy way to ensure that they will engage with your most important content.
Uses descriptive, relevant keyword rich anchor text in your sitemap improving the way search engines index and navigate through your website.