In the context of a writing group, a content warning allows the reader to steel oneself for perhaps some sort of intense content, or choose a different time to settle into a story. If they’ve had a day that’s already been challenging/triggering for them, they might choose to wait a day or two before reading. For the most part, I think people who are prone to triggers won’t skip things entirely unless they’re really going through a bad period. But some will. And that’s OK.
And no, you don’t get to critique a piece you haven’t read!