Simple answer: beats me. But it just does. Right now I'm printing out my novel-in-progress to review it as black ink applied to white paper. There may come a day when there is no difference, but after reading millions of words on the printed page before digital ever hit the scene, the difference remains there, for me at least. One of the little but important things I pick up on are the typos (not that I can catch them all) that jump out at me. Printing out a manuscript is expensive.
The toner ink alone is enough to break the bank. I saw an author who'd gotten a 100 grand advance (this was on a Booknotes interview) say he thought twice before printing out his long manuscript. I usually end up doing it 2-3 times for each project I do before its in the finished stage. Plus I like having an ink pen in hand to bear down and mark the corrections I want to make. I can't get that physical force done on a digital device.
This manuscript runs 261 pages double-spaced in 12-point type which rounds off to 66K words. That length is on par for most of my novels. So, I better get cracking on the proofreading. When the manuscript reaches the reading public, I'll be sure to let you know. Until then, thanks for checking out my blog and happy reading.