We recently visited a friend in Majorca where they’d had the coldest spring on record. Friend wanted to buy a couple of these - her very old village house had been seriously freezing.
So we went with her to a huge DIY/household store, to find some, only to be told, ‘We only have those when it’s cold.’
The weather had only just started to warm up a bit!
Many years ago these were our sole source of heat in Cyprus in winter, when we had marble floors and gaps under doors, etc.
Don’t know what on earth we’d have done without them. More than once I used my hairdryer to warm up the bed!