My USB wireless keyboard and mouse work just fine without vendor software, but if I ever lost the dongle and had to re-pair them with a different dongle, I'd need the vendor's software to do it.

My bluetooth headphones work just fine without vendor software, but apparently with an app I can adjust the audio to somehow make me better at playing computer games. I think it amplifies other players' footsteps or something? If I wanted that, I'd need the vendor's software to do it.

My PSU works just fine without vendor software, but includes a USB monitoring interface, which would let me see certain things like fan speeds, voltages and currents. Of course I can monitor most of those with my motherboard's existing sensors; and a dip in the 12v rail will power off the system before any monitoring could respond. But if I did want to use those features, I'd need the vendor's software to do it.

Despite my distrust for vendor software, I have even less trust for webusb. Partly that's because I'm a hater in general, but mostly it's because there are too many holes in the web browser's sandbox already - if things in the sandbox are re-flashing your keyboard firmware you've given up on sandboxing, you just haven't admitted it to yourself yet.