Customers often have no idea whether they want special requirements before they use it (or, of course, someone who knows what questions to ask can drill down on particulars). There's simply no way to have copy detailed enough to trigger a simulation of their business in their mind perfect enough for them to find every problem they are willing to pay to solve.

Your mileage may vary though, my brain may be addled by too much extremely high touch B2B work. :)