Even if you feel that your English is good, your text can always be improved. You have worked on your manuscript for a long time and know what you want to say, but it is difficult to see mistakes in your own text. Your brain knows what you want to convey and that competes with what you see when you read your text. An objective reader will have a fresh perspective, and a professional copyeditor knows what to look for.
In an academic setting, accuracy is a must. In research, attention to detail is taken for granted. A flawed manuscript will not be published. Not surprisingly, it is easier for writers with English as their mother tongue to have their papers accepted; the acceptance rate for non-native writers is much lower.
In business, correct English is just as crucial – language errors in advertising, in manuals, in customer correspondence or on a website will be noticed and will affect the company’s reputation negatively. That is why you need a copyeditor!