Towing in S6 or D?

Why do some people tow in S6, is D with Tow/Haul mode the right way to go? I have a column shifter

 

Welcome from WA!

The manual says tow/haul on and in S4.
I'll knock it up to S5 if it's flat ground or downhill.
Keeping it out of D lowers your transmission temps.

 

Easier to lock out of overdrives if it starts hunting if it's already in D6. Just flick the thumb switch.

On my truck anyway it will unlock the converter or downshift out of 6th at exactly the same engine load so it doesn't stay in top gear longer in S mode vs D. Whatever throttle it needs at 1800 rpm to reach 92% load on the scangauge is exactly when it shifts in either case.

Edit: ah, a '21. Use S4 when pulling weight. Fluid cook, many hots

 

On top of what's been already said, if you shift from D to S, even at highway speeds, it will default to S4. At least in S6, you can drop one gear at a time.