Quality procedures force those in my office to keep a record of their hand markups. We need to be able to document all of the checks that took place to get a document out the door...hand marked, initialed, and dated.
Then, when we kick something up for approval to go out the door, I need to print out fresh copy. Project managers also like their own hard copies to look at.
Basically, although I may not have a problem looking at documents on my computer, those responsible for the overall project do.