Read-only is the point
When you are reviewing someone else's work, or your own draft after a day away, you do not want to start editing. You want to see. A read-only view keeps the text stable while you read it, so the thing you are judging cannot change under your cursor, and every mark means the same thing it meant a second ago.
It is also the honest way to review a model's output. Marking the text without touching it makes the density visible before any rewriting starts, which is the number that predicts how much editing the piece will actually need.