Syncing files keeps them updated across devices and computers, so if you edit a photo on your iPhone, it will update the same file on your Mac. Syncing is about merging files to the most updated version across devices. A backup is when a copy of the files on your iPhone are stored in another safe location that could be a cloud service, the hard drive of your computer, or a flash drive.Syncing keeps files updated while a backup stores files for retrieval in the event that they are deleted from your phone.