I have used them, mainly FPS games. I also have built in functionality to change my mouse sensitivity in my mouse. There’s a “sniper mode” button as well that will change the sensitivity whilst the button is pressed to allow me to be more precise (not that I use that as I don’t play many games like that)

I used the separate x and y options when I was using a tracker ball once. It went slower vertical than it did horizontally so being able to change them individually was a huge help.

Other people that might make use of this feature (and perhaps even render a game unplayable without it) is disabled people, I don’t have experience here, but it’s not hard to imagine use cases, poor hand mobility, limited reach etc.