Hand valves are installed to balance the flow of water. This can be an experimental process, as I found out with my own zone valves, in a slightly different situation.
Since I "dump" my wood boiler heat into my first and second floor without the use of a thermostat, the second floor was getting all the heat because it was the last zone on the supply line. I had to practically close that hand valve to get water to go into the first floor, which is the zone before the second floor on the supply line.
In the end, the one hand valve is fully open and the other is almost fully closed, and now I have a balance of heat between the two floors.