There are lots of apps such as Whatsapp Messenger, ChatOn, and Google…, which needs to be synced with your contacts to improve the functionality. Cloud apps like Drop Box or SkyDrive needs to be synced with your device too.
By navigating through the Setting, You can easily specify the accounts to be synced.
Note that on some android devices, the syncing setting can be found under the Accounts and Sync menu.
You also have the power to choose which parts of the service you would like to be synced to your device. This feature is useful for Google Services, which for any reason you don’t want to have all the services (such as Calendar, Gmail or Picasa) synced to your phone or tablet.