Most of us treat our inbox like a storage unit. We open an email, think 'I'll deal with this later,' and move on. Before we know it, we're buried. People with clean inboxes get that every email is actually a decision waiting to be made. Delete it? Respond now? Schedule for later? Delegate it? They don't let decisions pile up because they know that unmade decisions drain mental energy.
Remember the Hans Christian Andersen story The Emperor's New Clothes? It's about an emperor who is convinced by some vendors' BS to buy a set of what's described as a beautiful set of clothes. There's only one problem; the clothes are imaginary. When the emperor wears (or actually doesn't wear) the clothes in a big parade, his constituents are afraid to say that he's wearing no clothes. Until a young child blurts out the truth: "The emperor is wearing no clothes!"