It wouldn't have to be complex, either, and it certainly isn't useless (says me and a few other people already!).
Quick example: I recently typed out about 10 long lines of business development chatter in Slack, the first of which contained a list of prospects. I realized I left a few out in my list, and edited the message to add them in. Without editing, I'd have to have either posted the complete list again or added them in as a second comment. Editing the original message is simply a lot less confusing.