This is a good question. Communication (writing and speaking) is essential in the business world. There are many reasons for this, let me give you three of them.
First, business is a team effort. In other words, in business you deal with people inside and outside the company. For this reason, communication is essential. If work is going to be conducted and finished in a timely and effective way, there must be clear communication on many levels. For example, the mission of the company, short-term goals, and the like have to be communicated well and often.
Second, the manner in which a person communicates is also essential. Part of the reason for this is because people make judgments based on a person's communication skills. So, to present yourself well will put you ahead.
Finally, all people are relational. So, the better you communicate, the better (usually) your relationships with people will be. This will help as well.