As the name implies, it cools the oil in the automatic transaxle. Automatic trans fluid can get quite hot due to the operation of the torque converter, the pump and the general lubricating and cooling of the trans components. There already is a cooler that runs into one of the radiator end-tanks, but it's pretty marginal. A good external oil cooler is a separate radiator unto itself.
Overheated oil is one of the leading causes of automatic transmission problems; hot oil tends to lose its lubricating properties (viscosity goes way down), leading to excessive wear and damage to the trans. Keeping it cool helps prevent this.
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