For example, let's say you want identical data in your Palm Pilot and your Yahoo! Address Book. While at home, you add John Doe to your Yahoo! Address Book, and later you add Jane Smith to your Palm Pilot during a meeting. At the end of the day, you'd like to get the same set of information in both places.
So you export your Palm Pilot data and import it into Yahoo! Address Book. You get Jane Smith in your Yahoo! Address Book but you also get duplicate entries for every other contact. To avoid this duplication problem, you could first delete all of your Yahoo! Address Book data before importing the Palm Pilot data. But then, since it only existed in your Yahoo! Address Book, you would have lost John Doe's entry.
Synchronization solves these problems by taking changes to your data, regardless of where they were made, and distributing them to all your contact managers. If you change the same record in multiple places, synchronization allows you to automatically reconcile the differences and take the most recent change, or to manually reconcile the differences.