There is nothing worse than a dozen clients calling because they can't use your products. A clear "user guide" shall spare hours of work for your customer support department.
A bad use or a wrong mounting can even cause damage to the brand new product you have sold. In this case, the responsibility of the manufacturer is engaged.
For some specific products ( luminaires for example ) the European regulation constrain the maintenance team to hold the mounting instructions in case of a problem concerning the product. You can find the last Euopean regulation concerning mounting instructions here.
20 years experience as industrial designer is certainly a plus for building good mounting instructions. Do not hesitate to contact me for an advice.