Springs are designed to absorb and store energy imparted to the vehicle by irregularities or "bumps" in the road. A very slow moving vehicle would need little more spring than what is required to balance the pull of gravity (the spring just holds the car up off bottoming), while a high speed vehicle needs enough spring (stiffness) to absorb the amount of energy it is likely to have thrust into it. You want your springs to be as soft as possible, which may be very hard in a race car, because if the spring is too stiff it would be as if the car were solid with no suspension. Too soft a spring is also bad, however, because it will simply collapse under load, again making the car behave as if it has no suspension. In between too stiff and too soft are the nuances that control stability and turning characteristics. softer front ends are grippier (often) and tend to understeer, stiffer front ends are more stable and tend oversteer. The reason people tend to overstiffen their cars is the stability that results; they usually soften the suspension when they realize they are sliding sideways too much and going forward too little.
I can explain this in simpler terms if necessary.
