You're looking at a book I wrote that was published in 2004. It is getting reprinted, along with the two other books in the series, so I took the opportunity to proofread it because I remember the first edition had many typos and errors. (In our defense, we were new at making books, publishing is much faster than most people think, and there is never enough time or budget. Not making excuses, just telling you how it is.) How many typos and errors? This picture shows a post-it flag for each thing that needs correcting.
Proofreading is a dying/dead craft. In fact, this book hit the presses without proofreading. It was edited. Twice. But not proofread. So in its current shape, the book is filled with enough orphans to fill an orphanage. And much more.
A proofreader who knows what he is doing can help not just spot typos but improve the flow of text by eliminating orphans and widows (not to mention orphaned lines and orphaned windows) as well as correct inconsistencies that the editor – who is chiefly concerned with grammar and language – may have missed. If you think that's trivial, you can join the rest of the human race as well as many publishing companies that don't employ proofreaders. But these people are wrong. Just take another look at the picture above.