Tiger puts a lot of power and intelligence behind its syncing services, but it doesn’t offer syncing support to meet all contingencies. When Tiger falls short, you have to jump off the Apple mothership and board a third-party craft.
There are many reasons why you might want to, ahem, sync different:
iSync shortcomings: Although iSync can sync Address Book and iCal information with lots of devices (see Table 3), you may have a nifty new piece of handheld bling that iSync knows nothing about; in that case, flip ahead a page to Add plug-ins to iSync for help finding an iSync plug-in for your device.
Or you may find that your device manages information that iSync doesn’t sync (for example, text notes, special ...