It is something to fight against the constant distraction of modern tech. It's hard to write a novel when social media is just a click away, your friends are chatting in your group, you're getting news alerts from everywhere and you even might fall into a wikipedia hole while researching about mongol tribes. So there really is a use case for devices who are only able to do one thing.

(it's kind of the same with phones: Get a dedicated camera and go on a walk and you won't be distracted be someone whatsapping you)