The use of showers for bathing in this country has been late in reaching these shores, due in part to cost of installing both a bath and a shower;-- so a thin-wall large volume tank of hot water, fed by low-pressure cold water from a header-tank in the loft was for most people, the only source of hot water, often heated by a "back-boiler", behind the coal fire. Electric showers use expensive electricity, and central heating has been beyond the financial means of the masses until the advent of double-incomes, which now seems to be the only way people can afford to purchase a home. Another point to be made is that drinking-water cannot be drawn from the tank in the loft, due to the risk of legionnaire's disease, etc.
You can blame the germans for that one (only joking) during the war it was better to use cold water storage cisterns as this would give famalies 24 hrs of bathing/toilet water if the supply was damaged. it is also a good idea if you are in a low water pressure area as only the kitchen sink is under mains pressure.
you can turn your system into a direct system but before you do get tests done first to see if your supply can cope. you only have to read all the "low water pressure" problems on this site to see the that even though its an old design it works well.