Hello: This idea was originally suggested by electric heater people to preheat their tanks, which tended to reheat very slowly. Old electric heaters came with much smaller heating elements than modern ones have.
This can work nicely if the “tempering tank” lives in a relatively warm place and has the insulation removed so it can warm up to ambient.
It also could give a nice supply of backup water in an emergency. I’d check the anode to make sure the tank lasts a long time. 😉