You have invested your time, knowledge, and possibly blood, sweat and tears in producing a document. You may have proofread it several times and found a few typos and other errors. However, for the final review you need a fresh pair of eyes to ensure your work is clear, precise, elegant, and error free for your audience. If your writing has grammatical, spelling and typographical errors, the reader may not read it all.
This is where a professional editor comes in.