No one is saying you should calmly explain to your child that they should move because they are about to get hit by a car as a Ford Explorer bears down on them at 40MPH. Get the kid out of imminent danger, and then have the conversation, obviously.
If you usually explain to your kids why you're asking them to do things, then a lot of kids are going to recognize that when you yell at them to get out of the street with no immediate explanation, there's some urgency to the situation, because you've earned the trust that you ask them to do things for a good reason. But if everything you tell them to do has no explanation, then it's just another thing you're asking them to do for no apparent reason, so there's no reason to take the request any more seriously.
"Because I said so" was always used in the rank-pulling, "I don't owe you an explanation due to who I am in relation to you" sense.