What's cleaner about this new approach? I can't see the point of it.

It's exactly the same as the User-Agent header we had, but worse.

UA was used for tracking? With this new standard, just ask the user agent to include all details in its Accept-CH header.

UA was used for feature detection? People will use this new standard to do feature detection.

And it's worse because there's legitimate uses of UA sniffing, and JS won't have access to it anymore - TFA wants to deprecate navigator.userAgent, so only the webserver would have access to user agent details? Why?