Affiliate system allow affiliates to earn a profit through purchases/subscription made by their clients. It encourages your members to share and introduce your website to others. It is a perfect tool to track referrals to your website.
Sitemap is a tool that inform search engines about the structure of your site. By placing a sitemap on your website, you enable Search Engine crawlers (like Google) to find out what pages are present and which have recently changed, and to crawl your site accordingly. You don't have to manually submit sitemap as it automatically (if settings is enabled in the admincp) Notify Popular Search Engines (Google, Bing) when sitemap changes.