Do you have 2 macs? For example, one at work and one at home. Then you are probably interested in being able to keep both those libraries in sync. Apple does not support a simplistic way of using your iPod to sync both libraries because of the possibility of someone taking their iPod to their friends house and copying all their music to their own library (in other words, they want to keep the RIAA happy by making it hard to use the iPod to pirate music). But there are legitimate reasons! Which I have illustrated above. Now onto the good part…
“iPod.iTunes evaluates the source for songs, videos and/or playlists that are not in the target and only adds these seamlessly, avoiding duplicates, saving your time. Unlike iTunes’ “auto update” function iPod.iTunes generally does not delete content in the target. iPod.iTunes can also transfer song dependend data like “My Rating”, “Last Played”, “Play Count”, album artwork, etc…”
So basically you can sync your home library to your iPod, then go to work and sync your iPod with your work library. Then all three will be in perfect harmony, iPod, home iTunes, and work iTunes. Amazing.