Sitemaps have 2 uses:
1) User Navigation. HTML sitemaps are created for the website visitor to give them a table of contents of your website.
2) Search Engines. XML sitemaps will communicate your website contents to search engines so they have an easier time indexing the contents of a website. If a website does not have good global navigation or linkage, the search engines might miss what you consider to be very important information on your website. If a website has a lot of media (images, videos), it is also a good idea to create an image and video sitemap. It makes a huge difference.