1. Everyone needs a proofreader
Even the best writers need proofreaders. When we spend so long looking at the same work, we're too close to it to look at it objectively, and our brain can misread things. We end up reading the words as we meant to write them, rather than how they are really presented on the page. It's always good to have a second pair of eyes take a look at our work - other people will pick up on things we have missed, even if we have read it through several times.
2. Mistakes make your work look sloppy
If you've taken the time to write something, whatever it is, you want it to be taken seriously. And a document that's riddled with spelling and punctuation errors won't be taken seriously - it'll just look unprofessional and it will give the impression that we don't care about our work.
3. Your spell checker doesn't catch everything
Sure, you can use Microsoft Word's spell checker function but it doesn't catch every single thing, for example if you spell a word incorrectly but it spells a different word (like if you miss the D off 'card'), Word won't pick that up because as far as it's concerned, the word is spelled correctly.
So whoever you are and whatever you are writing, if someone asks "Do you need a proofreader?" the answer should be "Yes!"
