This is particularly useful when editing the top-level comment of a popular issue to specify the current status. Or when a peer opened a placeholder issue and you fill it up. Etc.

If you actually use GitHub as a social network of sorts, there are many reasons to do edit comments. All the edits are visible anyway. You're on Git-Hub, you can already edit everything you have write access to.

Markdown is pretty tricky for new users to figure out, so quite often, users will just paste big snippets of code without formatting them, which is nearly unreadable. I'll usually edit these posts to add ```backticks``` around any code.

Censoring insults or illegal speech (depending on jurisdiction) would be the main reason I can think of.