One of the most common reasons why transmissions fail is due to overheating and high temperatures. This should be mitigated by the transmission fluid, although in some cars, this still isn't enough to help cool the transmission. For the most part, the majority of regular, day-to-day commuter vehicles out there could run find without needing any special transmission coolers. However, the same can't be said if you're driving a heavy-duty truck or are typically towing and hauling a lot of stuff. Driving with a lot of load will put more strain on the transmission, thus raising its temperature more easily. The same goes for race cars or hardcore off-roaders. In these instances, installing a transmission cooler would certainly help to extend the service life of the transmission by keeping it cooled.