DHW tank efficiency is vulnerable to sediment and it makes rumbling sounds when sediment builds up. The sediment grows depending on the hardness of water and other factors. The efficiency drops as heat transfer surfaces are covered with sediment. Flushing water heater every six months prevents sediment.
If the tank is clean, the thermostat reads the correct temperature and the heat transfer surface and heat exchanger maximize efficiency
Water softener prevents sediment but costs money and it also shortens the life of the tank
Savings around 2€ per month/extend life
References and best practices
Final report study on benefits of removal of water hardness (calcium and magnesium ions) from a water supply: