As I understand it (and I'm willing to admit I may be about to prove my incompetence), The oil pressure relief valve (#1 in diagram) bypasses the oil cooler with some of the flow when RPMs get high and the pressure rises in the system upstream of the oil cooler. The stock cooler is a flow restriction. There will be a pressure drop across that restriction all the time, but at low flow it's negligible. As the upstream flow gets higher at high RPMs the pressure rises before the restriction, and the pressure drop is greater across it. Without the bypass valve to allow oil to bypass the restriction the downstream oil pressure could drop too low to meet high RPM demand...a bad thing.
The oil pressure control valve (#2 in diagram) is the valve that controls maximum pressure system pressure by dumping oil out of the high pressure circuit altogether and into the sump. The function of controlling max pressure is why it's at the end of the plumbing system, not the beginning of it.