There are many reasons why writing in business is important. Let me give you a few reasons.
First, successful businesses are partially built on communication. Without communication a business cannot succeed and grow. Moreover, most of the communication, at least mass communication, is done in written form. Hence, writing well and properly is central to all business.
Second, if the goal of business is to reach as many people as possible with a product or service, this goal will be reached partially through communication. So, writing, once again, is central to the growth of businesses.
Let me give you an example of what I am talking about. Warren Buffet, the richest man in America, is a master of writing. His annual letter to shareholders is read by millions of people and they listen to his investing advice. His literally moves the stock market and tons of investors read his annual letter (actually study his letter). In light of this, we can say that writing is still the greatest method of communication in business.