The valve mixes cold water with the water from the solar boiler, which may reach temperatures up to 90°C). When no water is taken, the water in the pipe between the boiler and the furnace is filled with hot water, there is a high delta-t, giving higher energy loss.
Convection in the piping will considerably speed up the cooling down of hot water in the boiler.
Less hot water is taken from the boiler. The piping will only become 65 °C, therewith also reducing the delta-t and therewith reducing heat losses in the piping.
The amount of hot water taken from the boiler is less: if the piping has a volume of 1 liter , the boiler is 90 °C and we mix to 65 °C using 'cold' 20 °C water, 0.6 liter of 90 °C + 0.4 liter of 20 °C we get 1.0 liter of 65 °C. The other 0.4 liter of 90 °C can stay in the boiler.
Hot water will be trapped in the reverse-U shaped piping, as it has the natural tendency to go up, where it will stay put, in a very well insulated part of the system