There is one DPI setting per X server I think, and I use 192 because I have two 4k screens. Which forces me to xrandr the 1440p display at 2160p then output a blurry 1080p to match the 4k screens at 2x scale.
As for why use scaling, I can't read anything at 4k native and the UI elements don't scale correctly with the font dpi. Windows has fully functional 150% and 175% options (one of the few things Windows does well now) and in MacOS has really nice global super-sampling options between 1080p-1440p hidpi.
I am also using 192 dpi with a pair of 4k monitors, a 27-inch and a 24-inch, both at the native resolution of 2160p, so everything is crisp.
With the default font size set at 12 points, everything is very easy to read. Even 10-point or 9-point fonts are still comfortable, despite the fact that otherwise I need glasses for reading small printed books.
Under XFCE, almost everything is scaled fine on 4k and 192 dpi. The only exception is that some of the scrollbars remain rather narrow, but they are still usable.
As for why use scaling, I can't read anything at 4k native and the UI elements don't scale correctly with the font dpi. Windows has fully functional 150% and 175% options (one of the few things Windows does well now) and in MacOS has really nice global super-sampling options between 1080p-1440p hidpi.