Our Chevy Silverado has the button on the shift lever and it is used when towing. When I press the button a light lights
up on the dash that tells me it is in tow mode. What it does is allow the vehicle to stay in the gears for a longer time B4 shifting and keeps it out of overdrive.
The book says to use it when the truck shifts up/down to many times (Hilly Conditions). I do not need to use it on flat open roads where it will not shift back and forth so that is when I use the Overdrive.
Click and Clack stated in one of their articles that using the engine on a regular basis to brake the vehicle would not be good for the transmission, rear end and engine as it puts additional wear on these items and that it is far less expensive to wear out the brakes than drive train parts.
This is why having brakes on the egg is a major plus.
Click and Clack can B heard on the radio on weekends and are in a syndicated column in the Washington Compost each Sunday.
I personally believe that having the ability to use the engine to help slow the vehicle is a Major Plus when the situation warrants. That is the only time I do that.