Proofreading is extremely important in the business world for two main reasons: To demonstrate professional competency, and to prevent a miscommunication that can prove financially costly and highly embarrassing to all involved.
Business documents, including correspondence, are a central component of corporate operations. All office workers are expected to be able to produce documentation without grammatical errors or typos. That is absolutely a sine qua non of success in the business world, and failure to produce legible documents that are properly written and that have no such errors will likely lead to one's dismissal or, at a minimum, to one's failure to achieve promotions within the organization. Everyone, however, makes mistakes, no matter how experienced or competent. That is a given. The importance of proofreading one's documents, therefore, is in the opportunity proofreading provides to catch those mistakes before the document leaves one's desk. Professionalism is prized in the corporate world, and that means proofreading one's documents before submitting or mailing them.
The other major value in proofreading is to avoid potentially costly miscommunications. Again, any individual is capable of making a mistake in how he or she drafts a document. Improper use of a word or phrase hastily written, or a typographical error that fundamentally alters the meaning of the word or phrase, can cause the recipient of the document to draw improper or unintended conclusions. Such a chain of events can lead to one's dismissal, demotion, or suspension, and can result in serious financial losses for one's employer. By proofreading the document in question, the probability of spotting the mistake before it leads to a problem is greatly increased.