I mean, based on my interactions with marketing people, they often don't really know if much of what they've done has helped the business at all, and the majority of their work (apart from actually creating marketing copy and interacting with customers) seems to revolve around figuring out how to attribute booms in business to their previous campaigns while building plausible deniability for inevitable busts. Don't get me wrong, not ALL of it is completely incomprehensible: email and referrer links are pretty straightforwardly calculable in terms of their impact; but things like "brand awareness" campaigns are nigh impossible to actually gauge the impact of.