Even simple errors reduce the readability of a document and detract from your argument. Whether your document is a thesis, journal article, job or grant application, business case, essay or personal letter, mistakes can be embarrassing and expensive.
Not so long ago a colleague of mine wrote an impassioned letter to a government agency but omitted the word “not” from the sentence “this agency DOES NOT provide that service”. A document’s impact suffers greatly when its most important sentence presents an argument opposite to that intended. This mistake was certainly embarrassing, and very probably expensive with respect to the writer’s credibility.
My colleague’s letter (and cause) would have benefited from proofreading. Proofreading is the process of detecting and correcting errors such as typos, missing words, misspellings, inconsistencies, formatting problems and incorrect punctuation.
Note that editing and proofreading are not the same! Editing is primarily concerned with content, structure, clarity and logic of argument, expression and style; editing includes proofreading, which as noted above, is about relatively minor issues. Proofreading is a final check of a fundamentally well-written document, so speeds of 5,000 words per hour or more should be possible.