By my own experience, field studies are crazy useful for developing applications because you can study how the users interact with the current version or a competitor's app. It usually gives you tons of ideas on how to improve the features of the product.
Can ethnographic studies (or field studies) be useful for brochureware websites, i.e. sites that are only about explaining the benefit of buying a product? What insights can you discover (or have you learned) when visiting a potential buyer in her home or office that became a useful input to the solution that you wouldn't discover if you only interviewed the user over the phone for example?