The WebUSB API allows bridging of USB devices and the browser — they can communicate with each other. Allowing a USB device and a website to communicate with each other can open up several interesting possibilities and imaginations.
For example, consider the case of authentication for a website. A username-password combination is the most commonly used authentication method on the web. But imagine, you have been given a USB device by the website. When you plug in the USB device, you are automatically authenticated. Without the USB you cannot authenticate. This can be achieved with the WebUSB API.
This video was recently presented at JSConf and gives a nice introduction to WebUSB. It will show a very different view of what can be achieved with a browser — you can even build and control a hardware for it!