When you work in the publishing world, you hear a lot of 'oh, I wrote this book...' from perfect strangers. Many people have completed manuscripts on their hard drives -- perhaps the result of many years' research and pondering, or possibly the efforts of one inspired and frenzied National Novel Writing Month. Either way, these manuscripts are precious to their owners, and if the author has little experience with being published, she may not be sure what to do with her work.
The manuscript review service provides a professional eye to read over your words. It's a general but invaluable service to writers who want to know if their manuscripts have what it takes to get considered by a publisher or agent.
When a literary agent or acquisitions editor opens your manuscript, you have one shot to make the best possible impression with your writing. No matter how outstanding your story is, if the text is riddled with misplaced commas and awkward formatting, you're going to lose points quickly.
If you've got that one friend who's a wizard with grammar and has the time to pick through your manuscript, you're lucky. But many people want an independent eye to make sure their work is presented in its best light, and that's what a good proofreading will accomplish.
Like proofreading, polishing will make sure the world sees your text as it's meant to be. Polishing is for projects with a smaller amount of text: menus, advertisements, flyers, and other promotional materials. You want people to be talking about your new ad, but not because of that glaring typo!