Writing in English language is a necessary part of many professions, and most people take it for granted that if they can talk fluently, they can also write. This is not always so. Written English requires a solid understanding of grammar, punctuation, construction, and spelling - certainly more than a spell-checker.
The main reasons:
Errors in written material look unprofessional.
Mistakes in written material can cause misunderstanding for the reader. This can cost you money.
Proofreading requires specific technical skills and knowledge.
Mistakes are more difficult to detect in your own work so the only way to catch these errors is for a second person to check the work. (dual control)
