Buffer tanks accumulate cheap energy to defrost the pump. This is a common phenomenon at temperatures below 5oC with high humidity.
With the buffer tank the function of defrosting is not perceived by the user as it continues to provide ready hot water for heating which is already ready, stored in the tank.
Buffer-less heat pump installations (with inverter or not) often use built-in electrical resistors. Of course with much higher power consumption and significant defrost delay.
At the same time, it greatly reduces the total operating time of the unit, resulting in a corresponding reduction in energy consumption. Of course this also leads to a reduction in operating costs per season.
Finally, with buffer tanks there is the possibility of accumulating thermal energy using a night electricity tariff with a lower cost per kWh. Accumulated energy is utilized on the day when the cost of electricity is usually higher.
With the offered economy, it depreciates its cost in just 1 year.