Language has its own structure (not unchanging, to be sure, but fixed enough at any one moment to serve as both a constraint and a resource). If you do not submit yourself to the conventional meanings of words and to the grammatical forms that specify the relationships between the objects words refer to, the prose you produce will say something — language, not you or I, means — but it will not say what you wanted to say. That’s only because your readers will not be inside your head where they might ask the self-seeking expression what it had in mind, but will instead be on the outside processing the formal patterns of your written language and reaching the conclusions dictated and generated by those patterns.
In fact, however, what I’ve just said is a bit misleading because it suggests that fully formed thoughts exist in some inner mental space and manage to make it into the outside world when they are clothed in the proper syntactical and lexical forms. But as everyone used to know before the cult of self-expression triumphed, the ability even to have certain kinds of thoughts depends on the prior ability to produce (and comprehend) certain kinds of sentences.
People don’t think naturally in the future perfect or in parallel constructions or in the subjunctive mood; rather these grammatical alternatives are learned, and learned with them are the ways of thinking they make possible — relating to one another on a time-line events or states of being that have not yet happened; lining up persons, objects, and actions in relationships of similarity and opposition; reasoning from contrary-to-fact assertions to assertions about what was or could be done in the past, present, or future.
These are complex mental actions, and students will be able to perform them only if their minds are stocked with the right grammatical furniture, with forms that have no specific content but make possible the organization of any content into temporal/spatial arrangements that suggest and make available modes of action in the world.
The organization of the world in ways that expand the possibilities of thought and action — that, not self-expression, is the purpose of writing, and it is preeminently a social purpose. That is, it is a purpose not pursued alone but in conjunction with others to whom one writes (in speeches, essays, letters, memos, directives, proclamations, editorials, books) with the intention of imparting information, or clarifying issues, or establishing truths or bringing about changes or rousing armies or quieting conflicts, or any of the other ends one might work for in the public arena.
Writing then is, by and large, an act either of communication or persuasion, and to engage in it successfully, you have to do more than have something to say; you must be prepared to back it up, supply evidence, respond to objections, expose contradictions, parse the arguments of the opposition and so on. You must conceive yourself not as a lone voice singing in the shower, but as a participant in the multiple dialogues that are the vehicles of discursive and political life.