Perhaps the most important contribution a business analyst can make to the success of an IT project is to define the problem to be solved and determine what constitutes a successful solution to the problem. A simple question of the sponsor or customer or even users should accomplish the task. "What will you need to see or what will you need us to do to prove that we have solved your problem?" If they don't have an answer to that question, perhaps they don't really know what the problem is, or maybe they don't really have a problem, or at least one that you can solve. When we do get the answer to that question, we have the basic "why" of the project, and what our ultimate target is.
I believe business analysts are more than stepping stones, interpreters, translators, emissaries, documenters, or glorified administrative assistants. i believe business analysts are business problem solvers and the solutions do not necessarily have to be IT solutions (e.g. process improvement solutions). The outcome of a business analyst's work therefore is a solution document stating what the solution is - what the business needs to do to solve the problem, and it may or may not be a requirements document, or the total solution involving the change to business processes may include requirements.