the point of the sense of purpose step is to move from a general sense of why you are producing a work to a more specific goal. At first you probably won't have a very specific goal in mind, but as you work on your project you will gain insights into what you are trying to accomplish. Keep returning to this step as you go and push yourself to be as focused as possible. Remember that if you can't find a clear, specific reason for what you are doing, then it shouldn't be done, and your work will not be strong as it could be.
Ask yourself these questions to help develop a clear sense of purpose: