Why do balancing Valve installed in Chilled water return line for AHU?

when a valve closes suddenly at an end of a pipeline system and a pressure wave propagates in the pipe. It is also called hydraulic shock and it creates the water hammer. This pressure wave can cause major problems, from noise and vibration to pipe collapse.

Because the balancing valve is a multifunctional valve , that it's operation relies on the differential pressure across the terminal unit (in this case the AHU), so the pressure of the chilled water pipe after the AHU is needed as much as the pressure of the chilled water pipe before it. 

we have on the supply valve for balancing ....and to avoid water hammer .