Before I had back surgery in '94, I rode for 12 years with a plastic insert in a wrap around girdle that was under my clothing. It was called a Warm Form. When fitted, the plastic was heated up on a pad, then laid onto my back to take its exact shape.

With out this device, I would have missed years of riding. Here's the patent on it:

http://www.freepatentsonline.com/4572167.html

Those of you who have never had a bad back, count yourselves lucky. The rest of us used what worked to keep us going.

 

Maybe it's for when the fold up their bike and stick it in their backpack.

 

The guys back in the day who rode hard-tails wore them. Sit on one for 20 minutes and you'll know why. There was nothing 'fashion' about the things...pure and simple function.

 

More than likely it is for back support not for lifting weights. A person has had a back Strain/Sprain injury or blown disc and this can help with some of the pinching of the discs by keeping the back supported and in alignment

 

I tried a riding belt just once,the first good bump I hit it was ripped off and given away as fast as humanly possible.