[DanG]

The "stock" trans cooler is integrated into the radiator. You want the extra cooler in-line because the coolest the trans fluid can ever get in the radiator is close to the temperature of the fluid surrounding it- The coolant, which is usually ~200ºF.

The plate style cooler applies ambient air to cool the fluid. The theory here is the edge gets taken off in the radiator, then it gets cooled further in the plate cooler, then back to the trans.