Proofreading should be the final step before the book goes to the presses. A misplaced comma or missing apostrophe generally is more embarrassing than anything else, but for some publications, it could cost a lot of money to reprint materials. Oakhurst Dairy learned this the hard way when they neglected to use a serial comma (also called an Oxford comma or a Harvard comma). A missing comma in this case resulted in a loss of $5 million. http://www.cfodailynews.com/why-this-company-is-out-5m-over-a-missing-oxford-comma/.
If you've just finished the first draft of your novel, you do not want proofreading as your book could change substantially between the first and second (and third, and fourth . . .) drafts. After your first draft, you'd be best off with a beta-reader not a proofreader.
When you are ready to send off your manuscript to your publisher or to publish it yourself, you want a second pair of eyes to go over the text. You've become too close to your work and you need an objective expert to go over each word. As Amy Einsohn writes in her book The Copyeditor's Handbook a proofreader must "scrutinize each comma ('OK, comma, what are you doing here? Do you really belong here? Why?')" I will inspect each comma, question each pronoun, query each abbreviation, and investigate each semicolon. I can't promise that I will shine a light in the face of each period and ask it where it was on the evening of June 15, but I will make sure that it belongs there