Like if you're writing an essay on paper. You have your desk covered with a reference book or two, your draft version, working version, assignment, additional clarifications and notes, ... It would be such a hassle to put it all in one neat stack and search for whatever you need every time.

So programmers like to have the programming manual, design, notes, remarks from the customer, ... spread out over the screen(s) instead of switching back and forth every time.