Knowing who your audience is can be key because it forces you to understand what their role is, what is important to them, and how they like to hear information. By doing enough research and data gathering on the audience(s) that you are wanting to communicate to prepares you to then organize your thoughts in a way that most effectively can inform that particular audience of what it is that you want them to know.
For example if I am talking to my customers about the services I am offering to them vs. my suppliers who I am getting input to my services I would speak to them in two different ways. Who are your audiences? How can you tailor your message most effectively to each audience group?