Extremely good insight from John Siracusa (via Gruber) about why people tend to save files and folders on the desktops of their computers:
Quoting John Siracusa:
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.
Virtually every computer that I’ve seen or worked on over the past decade-plus, other than my own, has had files strewn about its desktop. This always drives me crazy. But looking at it from a non-geek perspective as John explains, it makes perfect sense: virtually every non-geek I know also has at least some level of difficulty with the whole concept of files and folders in a filesystem -- but the desktop is a separate, simple place unencumbered by that confusion.