Why? Because they provide clues of what may needs to be changed.
If you are an amateur writer or a business owner who writes your own content, you may not be aware of the kinds of corrections copyeditors make.
Copyeditors are well versed in using copyediting marks to do their jobs effectively when working with paper and pen. Microsoft Word conveniently uses Tracking Changes, but it is good practice to know what basic copyediting marks mean. This is especially true if you are receiving hard copy work back from a copyeditor,or if have aspirations of being one.
gives a short explanation about copyediting marks; a visual guide for handwritten marks is included.