Let's say you're selling a very popular textbook called Psych101.
Your search volume is very high for the generic name of the book, "Psych101" let's say 2,000 to work with easy numbers, and search volume is very low for something like "Psych101 eBook" search volume 50.
Low Volume, No Target?
How many SEOs will overlook the "Psych101 eBook" keyword because its search volume won't bring a ton of traffic?
Then they'll hand off the brief to a copywriter who follows along with instructions and makes little to no mention that there's an eBook available on their site. Maybe there's a UX button that a search engine can mildly detect.
Meanwhile, your SEO keywords "Psych101 eBook" and wins over those 50 customers who not only purchase Psych101 from your site, but they search YOUR site for other eBook titles rather than google. All of a sudden search volume plummets because everyone is just looking on your website instead of google.
(I think this is one reason why Amazon got so big)
Traffic < conversions
While you compete to rank first for the generic keywords, don't let easy opportunities pass you buy. The more loyal customers you win over in the short-term, the easier time you'll have winning larger campaigns long-term.