Proofreading should be your last stop before publication.
You’ve been through the editing process, and it’s now my duty to ensure your manuscript is free from any embarrassing errors. You’re SO close to submission—almost there! Now, I’ll give you a final quality check. Some of the things I’m on the lookout for include: typos, syntax errors, mistakes in grammar, punctuation, spelling, and formatting. Yes, you’ve read through it a million times; yes, you’ve run spell-check; yes, you’ve gone several rounds with your editor. But also, yes, your brain overlooks imperfections because you know what you’re writing already; and yes, spell-check is a useful tool, but it doesn’t catch syntax and formatting errors; and yes, your editor is awesome, but editing is entirely different from proofreading—and next to impossible to do simultaneously.
My goal is the same as yours: to present your audience with content they are one hundred percent able to stay focused on, while smoothly and easily reading through a flawless text. You want them to remember the substance of your writing, not balking at double-word typos, a misused homophone, or a forgotten comma.