When done right, lowering a vehicle can improve a number of characteristics, including handling, top speed, and aerodynamics. Having a lower center of gravity improves handling by reducing weight shift and the tendency of the vehicle to begin tipping over.
The two most common ways of reducing ride height are installing lowering springs and cutting the stock springs. Cutting springs is a much cheaper way to do it, but usually any benefits that would have been gained are lost due to reduced load bearing ability in the spring after being cut. Lowering springs are usually specifically designed for the particular vehicle, and provide much better performance than a cut stock spring. They are usually stiffer as well, to make up for the lost height and for a more sporty driving feel.
Other ways to lower a vehicle include redesigning the suspension and installing smaller wheels. A suspension structure redesign is quite an expensive route to take because it involves a lot of custom fabrication. Smaller wheels also reduce the ride height of the vehicle, but also change other characteristics, including tire/road contact, top speed, acceleration, and visual appearance. Additionally, every inch the tire size is reduced only lowers the vehicle half an inch.