Originally Posted by W.E.D.Jim

Springs are used to control weight. But they also control feel, with stiffer springs feeling like there is less time to react to track irregularities. Softer springs make the car feel lazier or easier to drive, but can also be too soft. For instance if your left front tire comes off the track in sweeper turns, the rear springs are too soft. If your car is unloading the inside rear tires in sweepers, or tight 180's as well, the front springs are too soft.

Shock oil controls the speed the chassis, or suspension can move the spring. A lighter oil car recovers faster because it transfers weight faster, but it can bottom out easier and be hard to drive on high bite tracks since it will feel darty in those conditions.
Even rough tracks can sometimes benefit from heavier oil/shock valving since sometimes it's faster going over bumps then it is keeping the tires on the surface at all times.

Still always about compromise.

Over the 30+ years I've raced I find that the people racing several hours and every weekend at major events are the ones who have the most time in testing and sort most if not all of this out for us...Yet we still search for that magic trick to gain an edge on our racing buddies.

It is important though to have a good grasp on why we do what we do and the trade offs for each.

Happy tuning!