Sitting at a typewriter, we are always in the present moment as the carriage trundles forward character by character, line by line. Word processing, by contrast, allowed writers to grasp a manuscript as a whole, a gestalt. The entire manuscript was instantly available via search functions. Whole passages could be moved at will, and chapters or sections reordered. The textual field became fluid and malleable, a potentially infinite expanse, or at least limited only by the computer’s ever-expanding memory.
(I would definitely agree that computers have changed the way a writer can relate to a book, and specifically, make it easier to alter individual facets of it, be they plot lines plot characters. I can't imagine writing a novel in any other way. Some writers I know, however, only write forward -- they don't go back once they've finished a page. Ever. Or at least that's what they say. It would be interesting to see a more in-depth study . . .)