OK....let's use the tea urn analogy. You can produce tea one of two ways; you can fill up the urn, get it nice and hot and keep it hot. When a customer comes in you have piping hot tea available immediately. Alternatively you can dispense with the urn and wait for a customer to come in and just heat up enough water to make a cup of tea for them.
To keep it simple, lets assume that during the day you use up exactly the volume of water in the urn. You will have heated up exactly the same volume of water using either method, to the same temperature (100C) so the energy required to do that is the same. But during the day, you also have to keep the water in the urn warm as well, so that's extra energy.
So why use an urn? Cos a customer wants a fast cup of hot tea. If you boil each cup of water it is more energy efficient but slower