Proofreading polishes clean drafts. Proofreading eliminates those teeny errors in spelling, punctuation, capitalization, grammar, and typography that the average person (and previous editors) wouldn't catch. It's the last stage of editing and requires eagle eyes. Spellcheck can't tell you whether you added an unnecessary comma or forgot one. Take heed of the classic example of the difference between "Let's eat, Grandma!" and "Let's eat Grandma!"