A transmission cooler is basically a supplemental heat exchanger that is located between the transmission and the transmission fluid / transmission oil reservoir.  A transmission cooler helps cool the transmission fluid.  

As my Impala SS does not display transmission oil temperatures, I can't report the first-hand differences in the transmission oil temperatures; however, Pontiac Grand Prix GXP owners who I've researched noticed a significant temperature reduction through the use of a Transmission Cooler.  Since T465E transmissions may be prone to failure, it's especially important to keep the transmission fluid as cool as possible.