Throughout the history of iTunes, iTunes sync includes backing up as the 1st step of the syncing process.
However, I have recently realised that this is not the case anymore. No idea when the change occurred, but this is a pretty significant change.
Backing up and Syncing are 2 different processes.
A back up creates a snapshot which ensures that you can easily restore your device to the particular state when you did the backup.
A sync simply ensures that the contents in your iOS device matches what is in iTunes.
Hence, if you only sync your device, you may have run into problems when you really do need to restore your iOS device =/