The factory cooler is in the radiator, does an OK job of keeping the fluid cool under most conditions. These coolers are added after the factory cooler. The fluid flows through the cooler in the radiator and then is futher cooled by the auxillary cooler.
Towing or lots of stop and go driving will push trans fluid temps up. I have read that contour trans are a little weak in the pump dept, and so can start getting warm on extended high speed runs also, these coolers help this. Your trans will generally last longer the cooler you can keep the fluid.
