Sometimes you can make a mistake, and with comment editing, you could make the comment how you wanted it to be

Sorry, but this has been rejected.

6.1 Delete or edit your comments everywhere
The inability to delete your comments everywhere is put in place so users do not post inappropriate comments or spam comments on other users' profiles, projects, or studios, just to delete them later on. In contrast, you are not really likely to spam your own projects or profile. The only benefits to this are to remove comments that you did not mean to post and to fix spelling errors. The former could be solved by thinking carefully before posting any comments, and the latter could be worked around by typing slowly and/or stating your mistakes in a reply to the comment. This also applies to forum posts; that is, a feature for Scratchers to delete their own forum posts will not be implemented. This suggestion extends to the ability to delete your own comments even if nobody has seen them yet.

Editing comments could similarly lead to comment manipulation, such as a user commenting “I like this project,” waiting until someone replies with “I agree,” and then editing the original comment to say “I hate this project.” Allowing Scratch Team members to view the edit history of a comment does not make it easy to moderate, since then they have to look carefully at the context of each edit, which takes time.