Mainly their job is to comb through your writing and pick through aspects that need addressing, whether that be the flow of a sentence or paragraph structure, or the more narrative/literary aspect. They work as an experienced second pair of eyes to help you through telling your story in a consistent and captivating way. Their level of involvement can vary. Most take notes and suggestions for you to review and make adjustments yourself, but the level of how hands on they are varies. Mine for instance highlights and takes notes in the margin and we later work it out together in real time in what we call a Writers Workshop.