What is the benefit of the de-coupler in the chiller plant?

De coupler is used to by pass the extra chilled water from the primary circuit equipped with constant speed pumps back to the chillers during part load conditions in multiple chillers installations with variable flow concept on the secondary side with pumps equipped with VFD's and two way valves on FCU's, AHU's etc.
Primary flow shall always be equal or greater than secondary flow to maintain constant chilled water supply to the air side equipment.
Without a De coupler in the system primary pumps could not be tested for their flow and head.

De-coupler is provided to bypass the chilled water from going to the AHU when the load at the AHU is less.
The primary loop will be having a constant flow pump and secondary loop having variable speed pump, so when the chilled water requirement in the secondary loop is less the extra water is bypassed through the de-coupler.