Actually, . .
It has a much more important reason for being there . . . whether directly before the pump, or anywhere else.
It, along with the airspace in in, allows the coolant to expand with rising temps and contract with cooler temps without putting undue stress on tubing connections.
AIOs manage that task by using either accordion tubing, or something with more than normal stretchiness.
A hard tubed build with no res or air space is going to be a problem when the coolant gets warm enough to have expanded by more than a few ml.