benefits of USB vs standard mixers

I have always used standard Behringer mixers. I currently have 2 of them - a Xenex 1202FX and a Xenex 1002. I am wondering, what is the benefit of USB mixers? What happens with the audio if it is fed back into the computer using usb? Is the USB kind of like a stereo line in on the sound card?
Not true. The USB mixer that I got (Behringer Xenex Q1202USB) was the same price as what I paid for my previous mixer which was a Behrenger Xenex 1202FX. It is also the exactly the same size. I was hoping that I could use the USB as an alternative to the line in on my sound card. But for some reason, there are no levels that are coming from the USB at all. So I have it set up exactly how I had my other one set up (through the line in) and in works great.