The illustration shown here is characteristic of a traditional system. Two water tanks tucked away in the loft, and a hot water storage cylinder.
One tank draws water from the mains supply and feeds the storage cylinder which, when heated by the boiler, can release gallons (or litres if you prefer) of hot water to taps all over the house.
The second tank, commonly known as F&E (feed and expansion) tank, is usually smaller and maintains the correct level of water in the heating system. It also allows for expansion of the water in either tank when it gets hot.
That is a real benefit of this system. Showers, taps and running baths can be drawn in any room at the same time; not all systems can do that. Handy, then, if a couple of you arrive home at the same time ready for a clean up. Although you also need to be aware that if the cylinder should run cold it will take a little time to reheat.