A proofreader provides a new pair of eyes to do a final check of your publication, to pick up any typos or howling errors that are likely to get between your reader & your message.
Proofreading assumes structure & grammatical checks have already been run, so a proofread generally won’t revisit these areas in any depth. It just picks up anything that’s clearly wrong & needs correction.
Have you ever noticed how easy it is to read words despite their letters being jumbled? If you’ve written copy yourself or read it several times already, it’s very hard to actually slow down your reading to carefully check things are as they should be. A fresh pair of eyes will read exactly what’s there, with no expectations & can often more easily pick up typos & errors.