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?