Written communication increases the possibility of communicating the same meaning to a large and scattered audience. If the written message is well written, so as to not be easily misunderstood or misinterpreted, readers will derive the same content from reading it regardless of their individual locations or situations. The key, of course, is that the message is written clearly.
Written communication demands thought and planning before it is commited to paper, computer, or whatever means of recording is used. That process allows for the crafting of the language, the development of expression or precision of wording that seldom occurs in verbal communication. The Declaration of Independence would not have had the magnificent language it contains if it had been delivered as a speech via Skype or other vocal means. Shakespeare's characters would not have presented the depth of emotion or the creativity of expression if the plays had been delivered without the benefit of a written script.