I recently read a book that was loaded with errors. Most were the type of spelling errors that spell checking tools might not pick up. For example, the use of the word “they” instead of “the.” Some errors seemed to be inconsistencies with the size font used in subtitles.
The effect was that I felt that the importance of the message in the book was diminished. (The crazy thing is that the book was about laying out pages in books and the author worked in the publishing industry!)
We know you don’t want readers of your books, website or company intranet to think less of you or your message because of typos. Good proofreading allows your readers to concentrate on your message.