The edit button would only be available for say 5 minutes after posting. Would show that it was edited with a history available to see what was changed. And according to Musk, could reset the like / retweets on it.
What I want from an edit button is just a convenient UI for deleting and resubmitting a tweet with small changes. Right now the UX for that is really bad - if I want to fix a typo I have to copy my tweet text, then reopen the tweet / reply box, paste in the text, edit it, and then re-add any attachments I had included (together with any accessibility text). And doing all this is even more difficult on mobile than on desktop.