A Definition of Purpose
Purpose is the reason why you are writing. You may write a grocery list in order to remember what you need to buy. You may write a laboratory report in order to carefully describe a chemistry experiment. You may write an argumentative essay in order to persuade someone to change the parking rules on campus. You may write a letter to a friend to express your excitement about her new job.
Notice that selecting the form for your writing (list, report, essay, letter) is one of your choices that helps you achieve your purpose. You also have choices about style, organization, kinds of evidence that help you achieve your purpose
Focusing on your purpose as you begin writing helps you know what form to choose, how to focus and organize your writing, what kinds of evidence to cite, how formal or informal your style should be, and how much you should write.