It's urgent. You've been writing, sweating, toiling and struggling. Finally you can press the send button and lean back with a wonderful sigh of relief... that is until the 1,234 recipients start calling one after the other, saying, "Er, I think there's a typo".
It's pretty hard not to make mistakes during a hectic day at the office. You've got thousands of other things to do, in addition to writing copy for that press release, email, product or brochure text. That's why it can pay to get a professional proofreader to run through your text before you send it off.
It makes you feel more secure and the recipient gets a more satisfying experience.