Regardless of the types of website you're working with, one things is consistent... you'll always need to provide some sort of content. With a sales letter site, where you're specially 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 different story. The task is and always will be to provide ongoing relevant information. At the same time, of course, you will be building the size of the website by creating additional subject-relevant (and hopefully keyword-rich) pages. 

Then there is the Adsense wibsite. Although it is a primary purpose is to gain revenue from targeted Google ads, content will always be a major factor in generating viewer traffic. 

See you on the inside!