Writing enables a person to put ideas into focus with more thought and clarity, because unlike speaking (unless reading a speech), we are able to take our time when stating what we want to communicate. We can be more selective in our choice of words. We can consider the tone of the language we are using. Whether we want to use active or passive voice. We can consider the moods we want to express: subjunctive, indicative, imperative--and there is talk of the infinitive mood. Also, we can rewrite what we have written without anyone needing to know what we wrote; something that the spoken word does not allow us to do.