Such a tank is needed to prevent backflow of water into the cold supply pipe,
No, the expansion tank is need to
allow water to flow back into the cold water pipe.
When cold water enters the water heater and is heated, it expands. That increases the water pressure in the household water system.
If you are on a well that increased pressure causes water to flow back into your well tank and the pressure in the house is maintained.
If your are on a municipal water system and there is no pressure reducing valve or the pressure reducing valve does not include a check valve, the increased pressure caused by heating the water in the water heater pushes cold water back in the municipal system thereby maintaining the desired household pressure.
If you are on a municipal water system and there is a pressure reducing valve and that valve includes a check valve, your household system cannot push water back into the municipal system. That creates excessive pressure in your household system until such time that a faucet is open relieving that excessive pressure.
If you are on a municipal system with a pressure reducing valve that incorporates a check valve, you need the expansion tank.
The expansion tank is a tank of air pressurized to the desired pressure of the water system. When the water pressure in your system is increased due to the heating, water is pushed into the expansion tank. You can compress air, you cannot compress water. This helps maintain the desired pressure of you residential water system.
