Sitemaps serve two purposes.
First purpose is to simplify the navigation of the website. A sitemap can be a list of pages in alphabetical order or shown in a hierarchy format.
Second purpose is to educate the major search engines like Google, Bing and Yahoo. XML Sitemaps should be submitted to the search engines to inform them that a website has multiple pages. If Google does not know you have mulitiple pages, they may not index all of the page. For Google, or any other search engine to show your pages in search results, they must be indexed. For them to index your pages, they must know that they exist in order for them to crawl the pages and show them in search results.
This is a very common mistake made by web designers. They launch a website and assume that Google knows that you launched a website. Tell me this! With more than 1B websites online, how is Google suppose to know that you launched a 8-10 page website. Not possible!
When we link to all of the pages on your website it will force the search engines to follow those links and index your pages. Additionally, links to your website help improve your image to the search engines suggesting that your site is relevant. The more links that point to your website, the better it will rank. Those links from sites perceived to be highly relevant sites, will help your website rank even higher.
Additional resources about Sitemaps: