Have you ever ridden over miles and miles of whooped out trail or through a rock garden, the kind that feel like your teeth are gonna fall out? Well, basically, that's when the kidney belt comes in really handy! ;)
I clipped this from a website on the subject for a more descriptive explaination:
"The devices themselves, vary in design, but are generally manufactured from layers of elasticised material, with Velcro ends for attachment and adjustment, and may or may not have vertical support stays for additional support. The belts are usually thicker at the back and narrower at the front and are stretched around the person’s lumbar region and waist area and fastened by Velcro at the front or side. This then is claimed to offer support to the lumbar area, by increasing the intra-abdominal pressure and supporting the spinal column, and surrounding organs. The kidney belts are used as a kidney support device in motor sport applications, such as off road motorcycle racing, and apparently help to hold the kidneys in place during rough terrain riding, and is supposed to prevent kidney bruising and blood in the urine, and are also non-specifically said to reduce pain."
I didn't wear a kidney belt for the first 2 years of riding because I didn't think I needed it. Then one day a friend took us on the desert ride from hell and we were on a really rough trail for some time. Rocks, whoops, the whole gamut, and at the end of the day my lower back was killing me. That's when I discovered the beauty of the kidney belt. I bought one right away after that trip and I haven't experienced pain like that again. :)