It's something that you could have derived if you'd thought to but didn't, like Bayes' rule. Once it's pointed out you immediately see why it's true and gain a fair bit of insight, but first you have to understand basic algebra. It's basically like clichés like "be the change you want to see in the world" but on a higher level; most normal people don't have enough knowledge to correctly interpret "be the change you want to see in the world", and most smart people don't have enough knowledge to correctly interpret "interpret every phenomenon as a particular dealing of God with your soul", but the few who do should benefit a lot.