Earlier quoted context omitted.
That’s exactly what’s rude about it. Don’t make sure you have their attention. Just send the actual message. If it’s urgent enough that the actual message isn’t enough, “Hello” isn’t going to cut it either.
I didn't make a value judgment on the practice, but it is a reason why you may get a "hello" message.
I've also had cases where I've immediately responded "hi" only to get the question about 1h later.