A fundamental problem in systems analysis and design is how to ensure an adequate specification. In a perfect world, the client will state what they require and the systems designer has only to determine the best way to build it. Practice is not so easy.
Clients commonly ask for one thing, while having in mind another.
What they ask for, what they want, and what they need are often entirely different things. Usually the client can state their problem only, in terms that they understand, at that particular moment. They may or may not have correctly identified the problem. They may or may not have expressed it accurately. They may or may not in fact understand their own problem.