In my opinion it’s not useless at all. Lemmy marks the comments as edited, but that’s just to show the fact that it was edited. But if you add the reason why you edited, that makes it a whole lot more transparent.
Sometimes it could happen that I see a great comment full of great ideas from a great user, and it could be lengthy as well. Then later I go back to see the reactions, and I see the comment was edited. If I don’t know what was edited on it, then I have to read the whole comment again. But if it’s clearly stated that only typos were fixed, then I don’t bother with re-reading the comment.