Moving a heavy piece of furniture is hard when one person has the bulk of the weight at one end. If the load isn't balanced on both ends, the weight's no longer equally distributed and settles in the front or back. Taking that heavy-ended piece around a corner and up or down stairs intensifies the problem.
A car or truck is weighted in much the same way. It has balance at pivotal points of the frame and across the suspension, and if it is lifted, the weight distribution changes. Leveling kits complement a lift or serve to keep the rear lifted higher than the front or vice versa. Trucks may tend down in the front when their cargo beds are empty, and a leveling kit will balance the end, creating an even, leveled look and better performance than with the drag of a lowered front end. Loading a car trunk or truck cargo bed with bags of sand is common for winter weather drivers, and leveling works on the same principle of keeping the height balanced over the road for safety and performance. Leveling kits often include spacers, which create balance when inserted in the spaces of coil springs. These kits vary in price from about $40 to $500 or more.