I am not sure why the valve is there but I have a theory. The filter is one story
below the pool and two stories below the solar panels. When the pump stops,
water flows back freely from the solar panels (or they would get scrunched
by atmospheric pressure). Without a valve, this might cause a backflow through
the filter housing and possibly back into the pool through the skimmer.
Having seen how grimey the water is in the dirty side of the filter housing,
I can see how this should be avoided.

Luigi