There are engine hot spots regardless of cooling method. There are variances in thermal conductivity. One reason red line coolant additive sometimes helps, improves the flow of heat into the coolant so it can be removed by the radiator. Similar thing with the oil cooler, just another path for heat removal from engine hot spots and get it into the coolant for removal by the radiator. Makes more of a difference when under heavy loads for long duration.
If you do a search for your name brand oil, you will see what I mean. You have to sift through all the salesmen chaff, however.