Regardless of the type of website you‘re working with, one thing is constant, you’ll always need to provide some sort of content. 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.