Battery grip for Nikon D3100?

Http://shashinki.com/shop/phottix-d3100-battery-grip-nikon-d3100-d3200-digital-camera-p-7729.html This product stated it includes the usb cable to connect to Nikon D3100 and D3200 camera body. Does this means this battery grip can only be used when the usb cable are connected to the camera? I never own any battery grip so I was curious as I thought battery grip can be used without any usb cable connected to anywhere.

It does.

Nikon did not make a battery grip for these cameras so there are no internal electrical connections for one.

Had you looked on the Nikon website under accessories for that camera, you would have noticed that. This is one reason many of us strongly recommend that you do a lot of independent research before buying a camera. Do NOT depend upon others recommendations or assume anything when spending $700 or more for a camera

The Nikon D90 or better cameras have optional battery grips available.

Spend some time on the Nikon site and you will see this.

There's no traditional battery grip for these cameras, so they have to fake it out by using the remote shutter release.

I'm not sure why anybody would buy a battery grip for these cameras. I've got one that is similar and I get a month or more on one battery.

It is not a USB cable. It is the wired remote shutter release cable so that the shutter release on the battery grip will work.

So, no - you do not need to connect the cable, but if you don't the shutter button on the grip won't function.