One of the best statements of need for proper Requirements Analysis is given in this course description by the University of Management and Technology in Arlington, VA:
"All projects arise in response to needs. In theory, the whole project effort should be geared toward addressing these needs. Unfortunately, needs are often incorrectly identified, poorly articulated, or simply ignored.
Requirements, in turn, are the physical embodiment of the needs. If needs are poorly defined, then the requirements that address them will be off-target, no matter how brilliantly formulated.
Ultimately, many project failures are rooted in the poor development of needs and requirements at the project outset. In this case, project work has not even begun when failure is hardwired into the project."
~ Introduction to professional development course PM013