The USB charger allows using the camera while it is powered through USB (other USB chargers don't seem to allow that). It is useful, for example, for recording a drive, in combination with the windshield mount.
While that sounds nice in theory, in practice it doesn't work so well. First, while this may charge the camera when it is off (I didn't try), it doesn't charge it while it is recording; in fact, the battery seems to be draining slowly if you're recording even if it is plugged into the charger. In order to use the USB charger, you need to leave the back of the camera open; that makes it difficult to push the on/off button (you need a pen or paperclip), and it also makes it difficult to remove the camera from the car mount.
In the end, it kind of works: you can record for about 8h at a time. But it's a fiddly and complicated experience.