Everything gets recycled and remodeled with today's fashion sense in mind.
According to PingMag's piece Haramaki! A granny item made fashionable, these Belly Warmers traditionally worn by seniors under their clothes have got a makeover. They can now be seen thanks to the company Hobonichi which produces two lines, one for summer and one for winter. Designs and fabrics are available only for that season which makes them a collector's item.
Haramakis are also said to be boosting your blood circulation by warming your belly and spreading the heat to the rest of your body.
According to e-budostore, "the Japanese believe that the belly ('hara') is the center of the human spirit and the source of 'ki' energy".
I do not see myself with one of these Belly Warmers unless it was made to look more like a Spanish style waist sash such as those worn by Flamenco dancers.
I have to say they are much better looking than ear muffs. You would not catch me dead (or alive) wearing these.