Love this bit from page four, on why people love to save files to their desktops:
The reason is simple: the desktop is the one “place” on the
computer that every user knows how to get to. People don’t even
think of it as existing in the file hierarchy (though, of course,
it does); to them it’s a location in the physical sense, and
items placed within it behave almost as if they were real objects.
A file can be “lost” in the file hierarchy — irretrievably, as
far as novice users are concerned — but finding something on the
desktop will never be any worse than rummaging through the
messiest real-life junk drawer. And that bargain, that task of
keeping things neat by placing, removing, and arranging, is
something that people are comfortable with, and that their innate
human abilities are tailored for.