I'll let TJ take this one. Ok I'll try. We lower our cars for different reasons. Some do it for appearance. Some to it for better handling. Some do it for both. Once the custom bug bites you you'll know it. I guess that about sums it up.
As I was "called out" . . .
Every spring design is a compromise. There is a balance of "soft" and "stiff" and a matter of height.
Lowering with springs is essentially reducing the distance between the body and the wheel center. Doing such also changes the geometry of the suspension, which is another factor affecting design.
For looks, but not performance, one WANTS s softer ride w/o the suspension bouncing off the bumpstops. That way, the ride is still good, but with a lower body.
For racing, one wants stiff. A track surface is (basically) flat and smooth, so stiff is good. But, a track suspension will beat you to death on a normal road. Also, a track suspension is often on the low end of a vehicle suspension desihn range, which may likely hit any speedbumps or bad ramp transitions.
For street performance, you want handling optimized w/ enough flex to take in street roughness and enough clearance not to drag when cornering or any road issues. Optimum may or may not be lower, actually, as it depends on the suspension design. Most suspensions are build on the upper end of their optimum range, so a bit lower is usually good. On a Miata, 1/4 drop is wonderful. On other cars, that is useless.
A well deigned performance spring has enough stiffness to reduce body roll and allow the suspension geometry to work like it should, while absorbing most road irregularities. The ride will be rougher than a system designed JUST to lower, but shouldn't be too rough.
The way companies reach each compromise involves spring length, coil number and spacing, metal composition, stiffness (spring rate), and several design elements (like making it progressive). There is a LOT of engineering behind a quality spring.
So, it all an issue of the optimum balance for intent.
(I've made several long posts on this topic, one recently-> on whether one should do springs or rims first. My search mojo just isn't working today.)
