Why do you need a proofreader?
Anyone can miss the simple things such as extra words, spaces, and homophones (The words that sound alike, but have different meanings like their, they're, and there.)
It takes time to keep up with all the rules.
Grammar, spelling, and punctuation mistakes are distracting, annoying, and can create confusion.
You don't want someone to stop reading your work, leave your website, or misunderstand you because of a mistake.
Anyone can make a mistake. Especially for all of my teacher friends and in honor of a new school year coming, here is a humorous example I found on Pinterest.