Give anyone a pack of post-it notes and they immediately know why they need and how to use it – even my 4 year old daughter took to it like a techie to open-source; and this is a product that came about by accident.
Similarly the SMS functionality of Cellphones was a “by-product” of the voice function. It was only after teens in Scandinavian countries started using it extensively that manufacturers and service providers saw the monetary value.
You can develop the best freaking product or service in the world, but if you can not make people see the value (see FAB)it has to them or they can not immediately ascertain the use of this product – the chances of success for this product is slim indeed.
You also have to pay close attention to consumers who actually use your, and your competitor’s products for that matter, they may just give you the “real” USP.