Good for use with a trailer, really heavy load in the bed, or I will occasionally turn it on when I'm hitting really curvy, twisty mountain roads, to stay within the engine's powerband without the tranny shifting constantly. Otherwise I leave it off.
it is so your wife can tow, too. I don't have to talk to her about engine braking, or any of that. She just pushes the tow haul and she is good to go. Now, she does more towing than I do. Horses, of course.