When all you want is marks
A full editor brings shortcuts, toolbars, paste handling and a document model, all of which can argue with the browser's own typing behaviour. Sometimes the job is smaller than that: you have text, you want to see the tells inside it, and you want to keep typing around them. This pad does exactly that and nothing else.
Because the marks are written into the content itself, you can click next to one and edit the sentence immediately. That suits a fast second pass, when the draft already exists and the work is trimming rather than composing.