"It took the users forever to sign off on the system."
Nothing encourages people to pay attention like asking them to take responsibility by signing something that says they accept it and all their concerns have been addressed. It is in everyone's best interest if acceptance is sought incrementally, rather than pushing it off to the last-possible moment. Work product descriptions encourage up-front consideration of what a deliverable will consist of and give people a chance to think it through before a huge effort is made to create a deliverable that people may not agree upon.
The Bottom Line
Work product definitions are your friends. At the start of a large project, it is often difficult to envision the details of the work products that will be created. The contract may outline deliverables and applicable standards, but as the work product looms on the horizon, it is often in everyone's best interest to revisit, amplify, clarify, and refine the work product description and seek agreement before much energy is expended creating the work products themselves. This provides an opportunity for negotiation or clarification to assure that everyone is on board and that expectations are managed.