Design diagnosis
A design brief assists all the key stakeholders in the design project. The designer must have every bit of information possible in order to develop an effective design solution. It is rather like a relationship with a physician. If the physician isn't told about all of the patient's symptoms, then the physician cannot offer the best treatment for the problem. Similarly, relevant information should be withheld from the designer, who needs it in order to design a useful solution.
A roadmap for the design process
For all stakeholders, including the designer, the design brief becomes a written agreement describing business objectives and the design strategy to meet those objectives. It is a roadmap through the process, a project-tracking document, an outline for a presentation for approval of the design project, an implementation plan, a plan for measuring results of the design project, and an archival document that will be useful for similar projects in the future.
The design brief should not dictate how a designer will actually execute the design. Rather, the design brief describes the problem and the desired business outcomes of the design work. It is up to the designer to create the most effective and creative design solution to solve the problem, using the most effective techniques employed by the particular design discipline.
In nearly all cases, the outcomes of the design project will be measurable data, such as an increased percentage share of market, an increase in sales of the product or service, increase in customer satisfaction, or increase in overall profitability. These measurements generally reflect the business needs of the enterprise.