Breeders use them to separate juvenile males who are aggressive. What I do with mine is use big shallow rubbermaid tubs which I fill with water and add a heater to it. I place the 32oz cups (which have lids + IAL, a small live plant) into the tubs (water level in the tub not over the top of the cups..). The water surrounding the cups is heated through the heater to keep them warm. Daily water changes are needed, yes, but it's still safe, warm and clean for them.
My breeding fish is the same way- but in a bit larger keepers and more plants/IAL.
(My pet bettas are in actual tanks and such in main living spaces)
Other breeders do it different ways, that is just what I've come up with to make sure they all will be warmed without having to use up so many outlets.
Unsure what the poster does, or whether they are a breeder or not.