Everything said so far is true but another added feature is it allows you to carry more weight with out having the stiffness (and rough ride) that it would actually take if the springs were up graded in a single set deal.
Basically they allow you to keep your nice ride (or semi-nice ride if you have a 2-ton truck like me) while empty but still be-able to haul a lot of weight without squatting the truck.
