@LukeAlmighty
Instant messenger type programs generally want you to write short sentences, one at a time, and send them instantly. So it will have enter send the message out.
But anything that wants to let you think carefully about what you say, and allows you to write longer messages, will probably use ctrl+enter.
@alyx
Just because of this feature, when I am at work, I do sometimes write my entire message in notepad before I copy-paste it to slack.