Do you know why many people love driving, doing extreme sports? One of the reasons is the necessity of staying very concentrated. The process is consuming all your attention, you should be right here, right now! The world doesn't change during these moments and it's still full of interruption, but your perception does.
Pen and paper have similar power and effect. When your business idea is only in your head there are hundreds of other thoughts which may interfere with it. But when you are writing the idea down all your attention gets automatically focused on it.
Writing down helps you to concentrate on one single thought, idea, plan. Each thought or project needs some proper space to be written on - that is why a concept 1 page = piece of content was developed.