Stiffer springs make the car feel more responsive, more direct. They also help the car jump a little better and higher. Stiff springs are suited for high-traction tracks, which aren’t too bumpy.
Softer springs are better for (mildly) bumpy tracks. They can also make the car feel as if it has a little more traction in low-grip conditions.
Stiffer Front Springs: The car has less front traction, and less steering. It’s harder to get the car to turn, the turn radius is bigger and the car has a lot less steering exiting corners. The car will jump better, and maybe a little further. On very high-grip tracks, it’s usually beneficial to stiffen the front, even more than the rear. It just makes the car easier to drive, and faster.
Softer Front Springs: The car has more steering, especially in the middle part and the exit of the corner. Front springs that are too soft can make the car hook and spin, and they can also make it react sluggishly.
Stiffer Rear Springs: The car has more steering, in the middle and exit of the turn. This is especially apparent in long, high-speed corners. But rear traction is reduced.
Softer Rear Springs: The car has generally more rear traction, in turns as well as through bumps and while accelerating.