All the categories you've listed have products that require a companion application to configure things out of band, that the "universal" driver doesn't understand.
In the case of the four HID you've listed the app would be for configuring key mapping, macros, rgb, firmware updates.
Some webcams need apps to control things not exposed by the native driver (things like head tracking or more specific sensor control).
I'm not familiar with the market but I would imagine that many headsets and DACs nowadays have similar apps to tune EQs presets and the like.
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.