If you intend to send a manuscript to a publisher, you should, at the very least, have someone do line editing. This goes for any type of manuscript: fiction, non-fiction, etc. The only exception might be a book of poetry where the author's creative license allows them to play with words, spellings, punctuation, etc. Professional presentations can also benefit from proofreading as well as copy and line editing, and this includes PowerPoint presentations. There is nothing quite so embarrassing as a major typo or an incorrect fact in a presentation.
Proofreading, then, is your first step in bringing any document to an audience, and one that most writers do as they go along. Furthermore, if you have time to proofread, you can always proof your own work, but simple proofreadingshould never be considered as your final step. Hiring a professional to take your work further, especially if it is a book-length manuscript, is absolutely essential if you want to put your best foot forward.
Here are some of the differences between proofreading and editing:
Checking for spelling, punctuation, grammar, word usage, and lower level formatting issues.
Proofreading + verifying facts, checking content and organization, fixing wordiness, awkward transitions. Can suggest improvements for clarity and flow.
Proofreading + checking for grammar, punctuation, spelling, consistency, word usage, as well as assistance with rewriting/rewording.
Proofreading + fact checking, checking for inconsistencies in character behavior/speech, style issues, readability, thematic variances. Content editors can suggest improvements in style and voice without obscuring the author's voice.