Answer:
Today's business world is almost entirely information-driven. Whether you run a small business or occupy a small corner of the org-chart at a massive multinational corporation, chances are that the bulk of your job consists of communicating with others, most often in writing.
Of course, there's email and the traditional business letter, but most business people are also called on to write presentations, memos, proposals, business requirements, training materials, promotional copy, grant proposals, and a wide range of other documents.