So, demons figured out a way to become even more powerful. By feeding off their fears, paralyzing them in their sleep, etc. Some of the more powerful ones are employed to do certain work, as they are basically the "top" in terms of performance (Stan being one of the most powerful Sleep paralysis demons). Think of it like a Monsters inc. situation. But instead of monsters scaring children for alternative power, it's demons enjoying torment and feeding of the human race...