> If the other party is away, and you leave before they come back, they can still answer your question, instead of just staring at a "Hello" and wondering what they missed.

Well, you can also just send a second message if your first one doesn't get a quick response:

Hey quick question

Just wondering when project X is due

Would this be slightly better as one message? Maybe in some situations. But some people would find it to be self-centered to jump right into "what can you do for me?".