In the few short hours since I started using #Threads, #DuckDuckGo has already blocked over 200 data tracking attempts. These include things like "headphone status" and "screen density."
Even if you do not provide a real fingerprint from your browser if you allow it to collect so many information it's still able to uniquely identify you because noone is going to have all those info the same as you
Screen density is usually needed to work out the resolution of images to load. A high-DPI ("Retina") screen uses higher-resolution images than a low-DPI screen.
opposite really... Notice "Screen Resolution" is on that list as a separate item as well...
Screen Density would tell you roughly how big the screen actually is. If you have a 4k panel... but it's only 6 inches, you can probably serve up a 720p image instead of the full 4k one and the user won't notice the difference since you'd have to hold the screen an inch away to see anything different.