Many writers are in their beginning, and they can't afford to pay a professional editor, and in this situation, they would choose us the readers, to give them our opinion about their work. If you would be a writer and need a review of your book, what you want ....an editor or the critical reader?
In general, a critique reader won't take your work to the same level as a professional editor, but it's a good start. Proofreading is only a tiny function of good editing. A professional editor will help you with story development, spelling, syntax, grammar, narrative.
Be aware, though, that a good critique partner should do more than proofreading.