Tow/Haul mode in simple terms keeps you in higher rpm's, allows for better control and allows trans to shift without hunting. You should always use it when towing, it protects the transmission.
Other options with cummins is engine break or exhaust brake, drove one for a decade and owned one, but I am not familiar with what the options are on your year. These protect you when doing down a grade mostly, but don't rely on them 100%, because going downhill and towing heavy you cant even be wrong once. Basically if a grade is that steep always use your gear box and the correct gear to keep speed low. These options will also save your brakes from getting too hot so you should use all of these features.