One of the major underlying issues people are calling out with the indictments of "smartphone culture", and trying to fix by going offline, is a sort of progressive shrinkage of the attention span. It involves progressively less continual attention paid to progressively more individual topics or items; the (current) apotheosis of this paradigm might be Twitter, where you read a constant feed of conceptually disconnected 140-character messages, and any longer-form thought needs unnatural gyrations of the underlying technology to express. Web surfing, or news reading, encourages this style of interaction, with the surfeit of links away to tangentially-related content that is usually not of a form to encourage deep immersion in a single piece.

Books, meanwhile, are of a form to encourage deeper immersion in a single input. Generally speaking, the obvious mode of interaction is to read a single book from start to finish, taking at least hours during which you remain focused on that topic. This kind of long-term focus is required to actually accomplish anything; therefore, a mode of information absorption that trains long-term focus is more effective of personal cultivation than one which encourages mayfly attention-hopping.

Nothing about this necessarily disparages the Internet; it's quite possible to focus on long-form pieces that are Internet-published, or to hop from topic to topic using offline media. But because friction costs encourage longer-term focus when offline, going offline is one effective way to incentivize it.