Is proofreading really a necessary part of publishing a book? Yes, yes it is. But not everyone needs to hire a proofreader. Why?
Let's start with the basics.
What is proofreading?
As I stated in my post where I explain the levels of editing, proofreading is the final stage of editing that takes place after a manuscript has been edited and typeset. This is the quality control stage where any remaining errors are corrected. If you skip this stage, your book will likely go out into the world with some embarrassing typos and formatting issues.
For proofreading, the manuscript will be a PDF file. Most proofreaders work in Adobe Acrobat because they can see the proper layout of the formatted book.
Proofreaders have a whole list of things to check over:
Missing text
Typos
Running headers and footers
Page numbers
Table of contents
Margin size
Gutter size
Widows and orphans
Word breaks
Loose/tight lines
Front matter
Back matter
Checking over these aspects of a text is a specialized skill. Proofreaders are trained to use a specific set of symbols called proofreaders' marks to point out any errors that need fixing before printing.
Do I need to hire a proofreader?
That depends.
If you are planning to send your manuscript to an agent or a small press, you do not need to hire a proofreader. You may want to hire a copyeditor or get a manuscript evaluation done, but your manuscript will not be typeset before you send it out.
If you run your own publishing house, then yes, you need to hire a proofreader. If a book goes out into the world with tiny margins, incorrect page numbers, and word breaks on every line, your readers will notice and will probably put the book down.
The same thing applies for self-publishing authors. If you want your book to look professional, you need to hire a proofreader. I recommend hiring someone other than your copyeditor to get a fresh pair of eyes on the text; someone who hasn't looked at your manuscript before will be more likely to catch any errors.
Many self-publishing authors skip the proofreading stage to save money, but this runs the risk of publishing a book that looks unprofessional. Appearance has a lot more weight than most people think, and that applies to the inside of a book as well as the cover.
Get that quality control!