I'm not sure I understand. After boiling it starts generating heat immediately? If so, it seems like it wouldn't be much more useful than a hot water bottle. I guess it is an exothermic reaction which would presumably put out more heat than a hot water bottle.

But you can't get heat on demand later as with other hand-warmers.

I still use one of those metal hand-warmers that use lighter fluid that I have had for about 40 years.