The proposed edit goes into a queue where you can see what the new comment will be. The moderator can always go to the plant and see what the comment currently says, but I should probably update the moderation queue to show a "before and after" screen like we do with the databox proposals.
The edit doesn't need to be okay'ed by the original poster.
Typoes are the main reason to propose edits to a comment. Outright factual errors on a second reason. Beyond that, if there is off-topic information or stuff that simply doesn't belong, those can be proposed to be edited out.
