It's useful for storing cheap solar energy for use in winter. Sizing the solar collectors for winter would require them to be much larger. Basically, this lets 2.5 W of solar in summer deliver 1 W of solar in winter. For a high latitude where the change in insolation is higher than that, this is superior. In Hamburg, for example, insolation varies by more than a factor of 10 from July to December.
Put another way: if you oversize the solar to be adequate in winter, you have enormous amounts of surplus solar in summer. So why not store it?
> So why not store it?
Because 0.10 $/kWh(th) / 0.4 kWh(e)/kWh(th) is expensive.
That's the capex for storage capacity. It's amortized over the number of charge/discharge cycles. Divide by 20 or more. It's amazingly cheap, actually.