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.