Defining interface data may not seem like much, but in systems that often have hundreds of messages and thousands of elements this is a lot of monotonous manual work. Typically there are a lot of changes made to the data throughout the lifetime of a project, each change has to be communicated to all the personnel, a daunting task within itself, and performed numerous times once again. This recurring process is annoying, boring, and highly prone to errors.
This brings us to our next point, bugs.
Various studies find that most bugs are introduced at Specification phase of software development, furthermore, transcription, communication and oversight are the major root causes of defects. In addition, there is wide acceptance in the industry of the theory, backed by some research that the cost of fixing a defect grows exponentially as development progresses over development phase.