Lowering is achieved in 2 basic ways:
1. Springs can be modified (shortened) to provide a lower ride height, which results in less suspension travel--the car will hit the bump stop(s) alot sooner--not good, for alot of reasons. This also generally results in odd handling because the geometry of the front suspension is so far outside it's originally designed "normal" operating position--it's "sweet spot".
2. Knuckles with a repositioned spindle can be used to raise or lower a vehicle from its original design ride height. In this case, lowering the vehicle is achieved by raising the position of the spindle pin on the knuckle, which "raises" the wheel(s) into the body, effectively lowering the car. The suspension geometry is unaltered, suspension travel is maintained at the factory-designed specs--unless you REALLY want to "get down" with shorter than stock springs....there are some engineering issues, and the better suppliers of lowering spindles (knuckle is GM's name for them) have the issues pretty well covered, though I don't know if the current Bell-Tech spindles have any limitations--for example, can you install any of the aftermarket brake packages if these spindles are being used?
I have to disagree with SINSSTR--if you were to install lowering spindles, realignment, or at least a check of alignment would ABSOLUTELY be appropriate. When the front of the car is lowered relative to the rear, caster is going to be changed, albeit in a minor way--still, it will change and should be checked and/or adjusted. Casting differences can easily cause the alignment to be off. When changing a part as significant as the spindle--and not even with an exact replacement--it would be the wise decision to put the car on an alignment rack soon after such a major change.
If you're wondering why the factory didn't do it that way, springs are alot cheaper than re-engineered knuckles. Making the brakes clear on every application, with different wheel sizes, fender opening clearance, bumper height and other Federally-mandated requirements, visibililty, entry/exit difficulty, etc--there are many reasons a production vehicle isn't always "optimized" to suit personal tastes as far as ride height, how well (or poorly) the wheel/tire package is integrated into the whole car--and many other considerations, I'm sure.