It could be to supply extra power to the hub. Basically the USB can only supply 500mA of power so by using 2 ports one can supply 1000mA double what one port could power.

The Standard voltage for USB is 5V and a USB 2.0 port on your computer will give you 1A.
I checked on eBay, a good source of information on what is available and several 4 port powered USB hubs are rated at 0.5A per port.
The ones that quote specifications on their power supplies say 5V 2A.

There is little support for them as there is very little in them that can go wrong, and if they are fail, it is simpler to replace them. One i saw while looking was under $15, shipped world wide.

hi YOU CAN FIND THE SOLUTION FOR YOUR HUB
Warranty Period : Lifetime Warranty Integrated/Included Cable Length : 1.8m Belkin Pro Series USB Device Cable LED Indicators : 1 x Power LED (Green), 4 x Port Indicator LEDs (Green) Max. Load Per Port : 500mA Package Contents : USB 4-Port Hub, USB Device Cable (A-B), User Guide, Power Supply Power Supply Specifications : 5 V DC 4 A (Provided) Option to Expand/Cascade : Yes, via Hubs of this design (stackable) or other Enclosure Type : High-Impact Plastic Enclosure

I don't have the original power supply for my QU2RCT7 either, but most hubs use a 5v DC power adapter, typically with a 2A. (2 amp) rating. I'm using such an adapter with my QU2RCT7 and it works fine. Make sure the adapter you select is tip-positive (ie, the "+" terminal is the "tip," or center, of the connector).

CHECK THE RATING ON YOUR POWER SUPPLY BECAUSE SOME LAPTOPS WILL REGISTER THE POWER CONNECTION PORT AS A USB PORT AND IF YOUR POWER SUPPLY VOLTAGE IS TO HIGH FOR THE COMPUTER IT WILL REGISTER IT AS SUCH, "HUB-PORT" IS SHORT FOR MAINBOARD PORT ON SOME DELLS. UNPLUG YOUR POWER SUPPLY AND CHECK THE RATING ON IT COMPARED TO THE DELLS INPUT VOLTAGE, THE INFORMATION IS NEXT TO YOUR SERIAL NUMBERS AND LABELS ON BOTH PEICES KEEP ME POSTED... JON-T

Most USB hubs do not need an external power supply, except when a connected USB device draws more power than the computer/laptop USB port can supply. In this case an external power supply will need to be connected to the USB hub.
Normally this would be a 5 volt power supply, but make sure the polarity on the connector matches the power connector on the hub, check the label on the hub, sometimes it states to power requirements of the hub as well as the polarity on the connector. Normally the pin on the hub's power connector is positive (ie. +5V)

i have one of those hubs the ac adapter is output +5V, and current 0.2 - 2.5A, inside of connector is pos(+), outside is neg(-), input: 100 - 120 Vac and 0.45A Max

DC 5V

Dear John,
this USB hub don't need any external power, but you are right there is a power input connector comes with every Targus Usb hub. but these hub self powered . you can use this hub without any worry.

Regards

5V DC 3.7A center+ polarity Adapter
Jay........