Documents should be created for explicit purposes or goals that both the writer and the reader would readily agree on. Although there are many explicit purposes for creating a scientific or technical document, there are four general categories: to provide information, to give instructions, to persuade the reader, and to enact (or prohibit) something.
Make the explicit purpose clear at the beginning of your document in an abstract, an executive summary, an introduction, or all of these. Sometimes a formal statement of objective is called for. You may also need to identify the person, the agency, or the contract requiring or authorizing the document or research.
In addition to explicit goals, however, writers almost always write with unstated but still extremely important implicit goals in mind. Among the most common of these goals are to establish a relationship, to create trust and credibility, and to document actions.