Google sync lets you synchronize your contacts and calendar on your phone with google account.
It works fine with following mobiles
iPhone
BlackBerry
Nokia S60
Nokia standard
Sony Ericsson
Windows Mobile

All most all the famous mobiles except on Andriod

It is useful in case if you lost your phone or want to change your old phone, you don’t need to copy your contacts and calendar manually or by some trick.
Google Sync is capable of

1. Synchronize your Contacts with Google
2. Depending on your phone, get Google Calendar events to your phone
3. Make changes either on your phone or on the Web