Not all ones not mentioned in this thread yet. A secondary 19" monitor ties you to a desk. A tablet would allow reading wherever people typically read paper printouts (in the train, while sitting on a couch, etc.)

When I have a complex paper to read I will spread it out on the floor. A single page at a time is too limiting, I want to be able to see page 5 and page 12 at the same time.

You're not wrong. But in those scenarios, I tend to be off the clock

It was nice to be able to jot things down in the margins (math, not just notes). Provide yourself the couple steps the author skipped (to show how clever they are).