What is a requirements document?
A requirements document describes the features and functionality of how something must work. Or in simple terms, a list of what something must do before it's ready to be sold. In new product development, the requirements refer to the functionality of the product and how it meets the user's needs. Successful new products solve a need or problem that exists within the market or application it is being developed for. The product should solve an important unmet need, and the requirements document describes how the product achieves that.
Why are requirements valuable?
In new product development, requirements are written down in a document or system. Through the stages or phases of product development, the requirements are regularly updated as new knowledge is gained about how potential solutions address the user needs. Traceability between versions and to test, verification, and validation plans are accomplished through a requirements traceability matrix (RTM). This knowledge is gained through testing prototypes in simulated or real use cases that are designed to match the final use case. When requirements are updated after new information is gathered, they become a better description of how to solve the problem. Typically, some initial requirements that were thought to be important, become less important, while new requirements that were not previously known are discovered. Over the course of several design and test iterations, the requirements document becomes an accurate description of how the product must work to solve the problem.
How requirements are more valuable than the design?
A requirements list is not written to be prescriptive. If it was, it would be redundant with the product design documentation and specifications. Rather, requirements are written to be sufficiently vague so that designers and engineers have creative space to come up with an original solution that satisfies the user needs as they best see fit.
Here is where the value of the requirements document lies. I believe that requirements are more valuable to the product stakeholders than any particular design is. When developed and written as described above, the requirements document can be taken to any design team and new or better solutions can be created.
For example, a company may develop the first version of a new product design based on an initial list of requirements. Following this, an internal team develops a prototype design and tests the prototype. During testing, it is identified that additional technologies are required to best meet the customer's needs. The requirements should then be updated and the project team will look for engineers within the company who have experience in the newly identified technology. In many cases, a company may lack sufficient experience with all technologies required and may decide to bring in an outside design team. The outside design team is given the last updated requirements document, the designs of the prototype, and the test report. Because the requirements document describes what the product must do, the external team is well informed to recommend improvements to the prototype that 1) resolve the missing technology, and 2) still meet the known performance requirements.
In summary, the requirements document is an essential component in any new product development initiative. Well-written and current requirements documents are more valuable to the product stakeholders because they represent all of the accumulated learnings obtained and describe how to solve the identified user needs. They can be given to any design team to develop an innovative new product design.