You’ve spent a lot of time on your writing, it’s finally finished, and you’re finished too. When you reread your project, you tend to see what you meant to write, not what’s actually written. Spell check will find some of your errors but can miss a lot of mistakes too. You can try the trick of reading it out loud, but this doesn’t always work.
Then there are all the grammatical errors which may be beyond you. Do you know when to use a semicolon, or when you need to use a comma. Are you writing in a passive voice? That’s something that you want to avoid.
In all, there are a lot of mistakes you can make and not even be aware of. Like ending a sentence with a preposition as I just did. You can get too wordy; you may have run on sentences, you may need to break up your paragraphs.
You have laboured over your writing, and you want it to be the best it can be, and that’s why you need to have someone proofread it. Catch all the punctuation and grammatical errors you’ve made.