I don't get it, how do you warm the water? I know you can have an electric heater that keeps a certain amount of water hot all the time. But isn't that vastly m

I don't get it, how do you warm the water? I know you can have an electric heater that keeps a certain amount of water hot all the time. But isn't that vastly more wasteful than just quickly warming the water you are using by burning it using gas?