With a sales letter site, where you’re specifically promoting one particular product, your focus will be entirely on causing the viewer to ultimately make that purchase. In that case, killer sales copy is the only content required.
Content driven websites are a whole different story. The task is and always will be to provide ongoing relevant information. At the same time, of course, you’ll be building the size of the website by creating additional subject-relevant (and hopefully keyword-rich) pages. Then there’s the Adsense website. Although it’s primary purpose is to gain revenue from targeted Google ads, content will always be a major factor in generating viewer traffic. Not just from the start but as a continuing process whereby additional traffic and revenue are achieved. For the most part, there are three specific categories of content… sales copy, articles, and RSS feeds.