I don’t fully agree with these, but these are the cases I’ve heard of:

• Deeper integration with webcams
• USB authentication devices like Yubikeys

I think these are better served with extensions or specific browser protocols that communicate with native apps in order to keep the crazy web world more isolated from the high-value computer world, but what do I know? My guess is that someone at Google went “You know, we’re creating a lot of these specific protocols to communicate with webcams, printers, and now we want to do authentication dongles. You know what? They all use USB? Why don’t we just create a general way to access USB?”

In the immortal words of Dr. Ian Malcolm:

There’s the topic of this conversation, WebUSB. I happen to believe that a missing feature here for Firefox is a good thing, mind you…