You need an unbiased editor because your mother loves you and she is going to love your writing because it came from you. Unless your mother or best friend is a professionally trained editor, they will not be equipped to edit your writing impartially. A professional editor will discern things in your documents, manuals and manuscripts that you are just too close to notice. You can do several drafts, reviewing and revising your words, but your familiarity with the work means that your mind tends to register what should be on the page, rather than what is actually there.