Everything listed changes way too often to be useful for tracking. My guess is that it's for anti-fraud purposes. Someone setting up fake devices and/or device farms is likely to get similar values, which means they can be detected via ML or whatever.
> screen brightness, memory amount, current volume and if I'm wearing headphones
None of those are likely to change when you navigate from one website to another, with tracking/ads disabled, which is what they want to be able to track. Otherwise they'd just use their cookies.
One device visits a site where you sell ads. A minute later, an unknown device with identical battery, volume, headphone, brightness, model number, browser version, and boot time to the second arrives on another site you run ads on. There's a pretty good chance they're related, because the odds of all those being the same plus those two sites and recent timings involved is rather low: https://coveryourtracks.eff.org/
Plus it doesn't have to be perfect. It just has to be good enough in bulk to sell.
