Having your work proofread acts as a final check before submission and helps to fix any errors that have not been spotted by the author. It can sometimes be difficult for the person who has written the document to spot any mistakes or inconsistencies, therefore, having a fresh pair of eyes go through it is always beneficial.
Even though many people have access to spell-checkers on their computers, these programmes do not always pick up spelling or grammar mistakes. For example, you may have spelt a word correctly, but it may be the wrong word completely. There are now also online spell-checker and grammar-checker online programmes, but they do not offer the same personalised one-to-one service that human proofreading and editing offers.
A misused word, or incorrect sentence order can completely change the meaning of what you have written. Therefore, those who read your document may become distracted from the real meaning of the content.
For students, good grammar, spelling and punctuation are vital; teachers will always mark down work if it is obvious to them that it has not been properly proofread and edited. Alternatively, a business colleague may judge the content of a document based on its poor grammar or spelling.
Essentially, proofread work is better work. All manner of errors and inconsistencies will be highlighted and fixed through proofreading.