You can flash GrapheneOS on a Pixel from another pixel, no pc required at all. I've done it several times, this is what sold me on the utility of WebUSB. You can use GOS' own distribution of chromium, Vanadium, if you have a GOS device and you want to avoid Chrome.
Is there something specific in that process that required WebUSB vs just normal USB? Sounds like phone makers could have done this since forever if they wanted to, what makes WebUSB particularly useful for this?
Native android apps can talk to regular USB devices, if granted the necessary permissions. But it's exposed through a Java api (and Kotlin I suppose, these days), which is fine, but it means you need to write your client logic twice. If you target the web, you can do it once.
(Yes, you could try to bulid some common interface, libusb-style, but I think you'll have a bad time with minor behavioural differences, especially around permissions. libusb itself does ostensibly support Android but there are several caveats: https://github.com/libusb/libusb/wiki/Android#does-libusb-su... )