Transmission fluid has many functions. The fluid is used to put pressure on certain clutches and plates inside the transmission which causes the motion of your vehicle. It is also circulated through the radiator to cool the transmission.
Over the life of a vehicle the transmission fluid is put through large amounts of abuse. Whenever transmission fluid breaks down it loses its ability to cool the transmission properly. It will also pick up debris which will be circulated through the transmission causing damage to the moving parts inside the transmission.
A transmission flush will remove the debris and gunk build up inside your transmission and radiator. It will also replace the old fluid with new fluid that will protect the moving parts inside. Just changing the transmission filter and refilling the pan will only remove about 25% of the old transmission fluid leaving a large amount of fluid inside the torque converter and radiator. Protect your transmission with a transmission power flush.