A statement of purpose is a brief statement (one to three sentences) of what you propose to write about and helps you to focus on the beginning of what you should consider in writing a paper. It should include your purpose in writing, who your audience is, and what the context for the writing is.
Purpose
In considering the purpose of your paper, consider what your main claim is and what you wish to persuade your audience about.
Audience
To persuade someone, or a group of people, you need to understand them. Who are they? What are their characteristics? (Age, gender, class, ethnicity, etc.) What are their values? Why would they be interested in your claim or find it important?
Context
Generally speaking, much of the writing done in class is because it is required. However, perhaps now, definitely in the future, you will need to write, whether at work or for personal reasons. The context of the writing changes according to its purpose--professional or personal--and, accordingly, so does its presentation. For instance, the language of an engineering report will be more formal than an email memo to a coworker. In addition, reports may also be accompanied by a presentation that is delivered at a specific time and specific location.