Well, say you don't always want to have the podcasts that you've downloaded on the computer to be on the iPod (cause they take up too much space, or you've listened to them and forgot to delete, or you don't want to listen to all your podcasts this week or whatever). If your iPod automatically syncs, it replaces the podcasts for you, whereas if you manually update your iPod, you can pick and choose when and which to sync. (I'm only using this as an example of the difference)