Objectively, the world is full of things that ask for your attention - your phone is buzzing with notifications, you get emails where everything is urgent and if you are not answering it within 5 minutes, you get a timely notification from your IM client, with someone asking if you’ve seen their email. The inbox rule allowed me to set time aside in the day when I focus on answering email, for any questions, concerns, ideas and meeting invites. This is twice as important in meetings - think of all the times you’ve attended a call where you were checking email, or dealing with something else other than the meeting at hand.