It's a leftover from reddit. A comment would be marked as "edited" if you changed anything after posting it, and since some people are shitheads and change their comments to say completely different things after people have already replied to them it's just a way to inform people that the edit you made to your comment was just to fix typos and not anything significant
Lemmy even shows ninja edits. On Reddit you had a few minutes to get a quick edit in before it would be marked as edited. Lemmy gives the mark even if you edit within 5 seconds.
The only problem is if you accidentally include some personal information or other type you don't want to be out there and you've edited it out, you probably don't want it to be accessible.