I use Tow/Haul all the time when towing either my 5'er or cargo trailer for all the good reasons already posted. Even on flats I use it.
I couldn't imagine towing anything these days without a vehicle that has some form of tow/haul mode or another.
To answer your question on driving on the flats. It might keep your converter locked at the higher gears allowing the engine to hold the right gear suited for your driving conditions (wind, slight inclines/declines).
Leaving it on while you're towing even on flats might keep things optimal for towing in those conditions present at that time. If you turn if off, even the slightest change might make the transmission downshift or try to upshift when you don't want or need it to.