The stock trans thermostat is set for 168 degrees, not bad if it stayed there but when towing in hills or mountains it gets far too hot, like up to 220. Once you're out of the hills it doesn't cool down quickly at all and as anyone knows heat is the kiss of death for a transmission. If you have the 68RFE trans, you have a double whammy as it won't last long with heat.
I've replaced the thermostat and put in the bypass unit as well as a secondary B&M trans cooler with a thermostatic fan on it as I do tow. My trans did fail at 97,000 miles and had it rebuilt to a Stage II level, it runs on a normal day at about 140 degrees and when towing it now never gets to 220, it stays cooler and when it does warm up and I get out of the hiills it cools back down much faster.
Here's another plus to the bypass, you can now fully flush out your trans fluid when it's time the change fluide. If you have the stock thermostat in the truck it blocks you from flushing the stock cooler when you drain fluid.