Search engines are watching your website, though they can run a little slow at times. If you want them to stay up to date with what you post, .XML sitemaps are the best way to do so. Since with the controversial Panda update, having an up-to-date sitemap is even more important than ever, because it helps you prove that you are the original publisher of a disputed piece of content.
In theory, these files actually make a direct communication channel, always ensuring that crawlers get notified faster and more accurately about every content change that you make.
In practice, if your .XML files are not correctly generated and easy to crawl, your SEO score can bruise easily.