A large portion of our world energy use is to make heat. Thus if you can make the heat you need when there is plenty of renewable energy available, and then store it for use latter that is a large win. Sure we can't turn that heat back into electricity (false, but they are not worth talking about), but since heat is the goal that doesn't matter.
This is well understood. My parents have been on a off-peak water heating program since 1988 (in all those years they only ran out of hot water 5 times, and nobody was trying to save water). Based on that experience, just the hot water a family uses in a day is in the 300-800 liters range (go high - running out of hot water for the day sucks). Heating your house is a lot more though - 40000 liters is a low end estimate I've seen.
You won't be cooking food, powering your car, or lighting your house this way, but it is still a cheap and useful way to store energy. It is also something we can do for the world using yesterday's cheap technology.