I've never touched them. don't think i have to unless someone explains to me an advantage to setting the sliders up
Very fine control and reduction in nose bounce. It's hard for me to describe, but if you set your stick up right, you can get very fine control in the range of gun solutions and it helps to overcome the "break-out force" on some sticks like the X-52.
Personally, I'm a control freak and set my sliders to their full extent. This is because I've personally become very good at concentrating during a shot and move my stick in small movements, reducing nose bounce. People with less accurate sticks or that get too excited can set the scaling to overcome that for them. Basically, the higher your sliders are set up, the more "violent" your stick becomes. If you set them too low, your reaction time will be slowed as when you jerk the stick back, the input slowly swoops up after it.
I see the most effect in GVing. By using a sliding scale there, very very fine adjustments can be made. Much more fine than would ever be required for flying. Having a joystick that requires very little deadband also helps.