Your computer automatically trusts every device you plug into it. But every USB device is actually a small embedded computer that runs software you cannot control.
USB driver exploits work by sending malformed or unexpected input to your computer's USB drivers. Your computer likely has hundreds of USB device drivers installed, and a programming error in any one of them leaves you exposed.
The USG blocks these attacks by passing data through an internal serial link with a very simple protocol. Only a limited number of safe commands are accepted by the receiving microprocessor, so no malformed or unexpected data is transmitted to your computer. This effectively blocks USB driver exploits.
The USG is a firewall for your USB ports.