iTunes is needed because unlike a normal MP3 player, which rebuilds it's "library" (index) whenever you unplug it from a USB connection, the iPod needs iTunes (or any number of other programs that are iPod compatible (via plugins, usually), such as WinAmp) to explicitly update it's library. You can copy an mp3, but since the iPod doesn't perform directory enumeration (and instead relies on what is in it's index) the file is ignored.
They could have easily make the iPod do it itself, but then they couldn't force you to use iTunes, could they?
