People learn in different ways. The idea behind the shelf is to have many ways to approach the same topic so that I can tailor time with a person to their best learning style.
When all that is prepared beforehand it allows us to think and engage with the other person without much thought about what we are going to say. We can listen better. That may lead, and often does, as I mentioned yesterday, to putting what I was going to talk about back on the shelf and bringing something else into the conversation.