We all know Bob. He's that friend who reads constantly, so why shouldn't he edit your manuscript? What's the difference between him and a professional editor? They both read. They're both articulate.
All editors start out as voracious readers, just like Bob. However, as they gain more experience, editors become trained to read differently. Unlike Bob, editors do not read for pleasure.
An editor is trained to read uncomfortably, to question and probe, to demand more from the author. As soon as an editor begins reading for pleasure, he must stop editing. His enjoyment represents a distraction. He is no doubt missing mistakes and inadequacies in the prose. When this happens to an editor, he puts his pencil down and reads the manuscript for pleasure. Only after the reader in him is satisfied, can he return, pencil in hand, to do the editorial work.
Any author who has seen an edited page will know that editing is a slow, laborious way of reading. Comments and changes require time, concentration and above all, a critical eye.
In fact, once editors achieve a certain level of experience, it becomes almost impossible to read like the average reader again. Remember when the editor stopped editing to read the manuscript for pleasure? Well, he was still evaluating it at the back of his mind. It's a habit that, once acquired, cannot be reserved for manuscripts alone. Even at the beach, with his summer reading list, the editor is evaluating different aspects of the book's narrative and prose. He's paying attention to structure, character development, theme, pacing, and consistency of tone and language.
Something else most Bobs don't know, is that editing is not rewriting. Even when it's easier for the editor to rewrite a tangled paragraph, he restrains his pencil. Instead, if clarity cannot be achieved by restructuring, the editor must find a way to draw the words from the author.
Don't misunderstand: You should give your manuscript to a Bob. It's beneficial to have beta readers give you feedback. Just don't assume Bob can replace an editor. A professional editor's changes will markedly differ from Bob's.