It's also extremely important to have someone correct your manuscript for style and errors. Errors in your work will cause many agents or editors to pass on your book without really reading it, because errors or sloppy sentences signal an inept writer. However, that sometimes takes years. If you need help in one area, it almost always affects other areas. Once in a while a business will need a copyedit on marketing material or internal company documents, where the content is pretty much set.
Most writers prefer to have content, character development, plot, etc., looked at as well.
We request a synopsis and sample pages. We request a deposit of half the estimated amount. We write the critique together, carefully discussing exactly what we're going to say and how we're going to say it. This involves a second read of the manuscript. We both mark the pages and read each other's comments to be sure we agree.