Importance of Designing a Brief

What’s in a design brief?

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 must not be withheld from the designer, who needs it in order to design a useful solution.

A fully developed design brief for a major project must incorporate the background, needs statements, and tactical information such as time frames, budget, and desired outcomes, contained within both the RFP and the proposal. Once these two documents have been completed, they are melded together into a proper design brief, which will usually include more comprehensive data than had been included in either the RFP or the proposal. It is essential this document is written down rather than being in the form of a verbal agreement to avoid later disputes.
A truly useful design brief should be developed by two people - one representing the group with the business need for design and one person representing the design company that will execute the design work. Both individuals are equally accountable for the results of the design project.
A good design brief should answer the following questions:

• Why are we doing this project?
• What are we trying to accomplish?
• Why is this project needed?
• Why are we doing it now?
• What specific business outcomes, or results, do we expect from this design project?
• Who are we designing for (this requires a very precise and complete description of the target audience for the design project)?
• Who are the key stakeholders (eg sales, marketing, law, distribution, procurement, manufacturing, etc) in this project?
• What is the current and anticipated business environment for the results of this design project?
• What, precisely, are the phases of this design project?
• How much time must be devoted to each phase?
• How much will each phase cost?
• What is the competitive environment like?
• Who will approve the final design solution?
• What criteria will be used for this approval?
• How will the design solution be implemented?
• How will the results be measured? 

Example brief
http://www.graphicdesignblog.co.uk/sample-brief-design-packaging-for-a-box-of-dog-treats/