When you get an idea, so many things come in that one moment. You could write the sound of that idea, or the sound of the room it's in. You could write the clothes the character is wearing, what they're saying, how they move, what they look like. Instead of making up, you're actually catching an idea, for a story, characters, place, and mood - all the stuff that comes. When you put a sound to something and it's wrong, it's so obvious. When it's right, the whole is greater than the sum of the parts. That's a magical thing that can happen in cinema.