Sounds like something that could have a standalone usb-driver-container or special chromium fork for the 0.00001% of users that need it instead of bloating every browser with yet another niche API and the inevitable security holes it will bring.
Or some things aren't even available made using libusb. Think control applications for RGB lights in keyboard and mice. There's a certain manufacturer all but mandating installation of its slopware. Being able to provide all of this as WebUSB has advantages.