The most common reason is to make the cars appearance better. Many spring manufacturers will advertise the fact their sport spring makes a car handle better! This is strictly not true as a car lowered with springs and standard shocks, will often handle worse due to the standard shock absorbers being too soft and bottoming out during harsh cornering.
The relationship between the shock and spring is of paramount importance and fitting sport springs and no sport shocks is definitely not the ultimate set up. A sport spring is approx. 20 % stiffer than a standard spring and once again needs a shock matched to this for the ultimate performance.
HOW WILL THE RIDE BE IF I FIT A LOWERING SPORT SPRING?
The lowering of a car actually removes valuable movement from the suspension so naturally the car will bottom out sooner than it would at standard height. The factors to worry about are what condition are the standard shock absorbers? How low is the car to be? What length and what material are the bump stops made of? If shocks are standard, and or worn the ride will not be desirable.