I was a little confused at first myself.
But I think you get it and I do now.
If you plugged the OEM Blackberry charger into the back of the cradle you'd have to unplug the sync cable to do so. You'd then be limited to only charging the Blackberry. When you wanted to sync your device via the cradle you'd have to then unplug the charging cable and plug in the sync cable.
By using the DC in 5V cable you're able to both charge and sync and more importantly charge when your computer is off without ever having to unplug a cable.
The cradle claims to utilize some sort of "quick charge technology" so I'm guessing having two power sources plugged in won't have any adverse effect. Personally having owned and reviewed many of Seidio's products over the years I trust what they stand behind.