Comment by belviewreview

The thing about focused work, at least a lot of it, is that before you start making any progress you first have to remember a whole lot of specifics, like facts, goals, constraints, and so on, and how they all fit together to form the situation you are dealing with and what you are trying to do. And that takes uninterrupted time. If you are interrupted, it all evaporates from your mind and you have to remember them all over again.

It also takes mental energy, which everyone has only so much of each day. If you are interrupted, then all the mental energy you had spent is wasted, and you have less left for the rest of the day.

One more thought. I think the people who don't understand why interrupting is bad are people who they themselves don't know how to think in a sustained, focused manner on a difficult problem. And as a consequence of smartphones, such people are considerably more common than they used to be.