Why does the DC Boarduino have a USB-Ext jumper?

Why does the DC Boarduino from the ladyada website have a USB-Ext jumper? There is no USB port! I was going to get this board and run it off a 9v battery, but now I'm not sure if I understand the power setup right. It makes no sense.

The boarduino has a row of six pins at the opposite end of the board from the power jack. These pins are used to connect the Boarduino to your pc via the USB TTL 232 cable Ladyada sells on her website (http://www.adafruit.com/index.php?main_page=product_info&cPath=19&products_id=70. If you use that cable, you have to set the jumper to USB.

Have a look at http://www.ladyada.net/make/boarduino/. Ladyada explains how the DC boarduino works and why you need the cable.