Commonly known as user-manual. On the one hand the customer tends to ignore it, because it is hard to understand, badly translated and either of impressive volume or on loose pages. On the other hand engineers and sales-staff do not really take notice of it, as they consider the product self-explaining. So why should the customer see it another way?
For you, the manufacturer or reseller this has two possible consequences:
Firstly, the customer does not manage to use the product properly, and contacts your support-staff. This costs your money, your time and the nerves of the customer. This at best, as many customer surrender and send the product back right away. You have a returned product not being resellable as new, and probably one customer less.
Or it is the type of customer who suffers in silence. He does not send the product back, but is unsatisfied with it. He will not buy again, and he will share his experience.
A well done user-manual is clear, brief, without idiomatic errors and has understandable sketches. And it costs far less compared to what it saves.