The belt is worn to increase intra-abdominal pressure. This helps keep your spine aligned when straining on a heavy lift and makes it safer. A belt is also a reminder to lift properly while in the gym from loading plates to moving DB's around. I found this written by a G. Welch.
The weightlifting belt, contrary to popular belief, supports the back in a different manner. While there can be some support due to the rigidity of a weight belt, the benefit is minimal compared to the support offered by the increase in the intra-abdominal pressure (IAP). (8,9,10)
During a normal lifting maneuver such as the squat; the diaphragmatic muscles along with muscles of the torso contract generating pressure on the abdominal cavity. The abdominal cavity together with its predominantly fluid contents, known as the "fluid ball", are kept under pressure by the surrounding musculature. It is this natural intra-abdominal tension that supports the spinal column. (11) Several studies have shown that weight belts increase intra-abdominal pressure and therefore assist in this natural stabilization mechanism. (3,8,12,13,14) Other studies have shown that increased IAP by use of the weight belt not only relieves the load of the musculature of the lower back, but also reduce the compressive forces on the spinal discs. (15,16,17) Furthermore, it has been reported that the reduction in these compressive forces can be reduced by as much as 50% when utilizing a weight belt. (16,18,19)
