Like copy editing, a proofreader is the last point of call for errors. They should proofread a formatted work because part of their job is to ensure the formatting is consistent. They also check for grammatical and punctuation errors, but they should not suggest changes to words or wording (unless grammatical). When proofreaders are starting out, many proofreaders struggle to know when to stop editing. We are horrible at that ourselves, but our bottom line is affected if we do too much.
Remember, most books will have an error that no one spotted, regardless of how rigorous the editing and proofreading process was. If you are proofreading for yourself, consider using ARC (Advance Reader Copies) readers. Another set of eyes is fantastic.