This was the only one I could think of a good reason to track. I immediately thought of all the grandparents and tech illiterate people who'd probably implode if they had to pick .exe vs .deb vs .dmg/.app (I actually had to look up what MacOS uses...) vs etc. And don't even try to have them guess intel vs amd.

Automatically guessing the operating system saves us tech people from having to figure out they downloaded a file for a completely different OS.

Operating system and CPU architecture are useful for sites to serve the correct binaries when a user is downloading an application.

Barely. You could trim down the data to incredibly low granularity ("OS: Windows", "CPU: Intel Desktop") and you'd still get the exact same binary as 99% of the people 99% of the time, anyway.

Oh yes, that's a very good point, actually. That actually seems such a fundamental use case that you could almost justify it being available without a permission.