That's a common myth that many people think.
With an adequately efficient radiator, when the car is moving, coolant temp should run at or near the thermostat rating. The Tstat is actually starting to close, or remaining in a partially closed state. It is doing it's job of keeping the temperature warm enough by controlling the flow through the radiator.
Now, when the car comes to a stop, and there is no airflow through the radiator, the Tstat fully opens, but despite that, the temperatures continue to climb, until such a time the fans turn on and bring some air through the radiator.
Now, here's where the two components tie together. We've been told by many engineers and experts that the LS engines have a "sweet spot" of optimum coolant temps in the 180-195° range.
With the stock thermostat, you probably find your temps while moving hover right around 192°, right in the range you want to keep it at. But what about the fans....they don't trigger on until at least 219° (depending on year of car). So we want to reprogram the fans to come on sooner, and not let it get so hot, that's easy...but you cannot cool it down more than what your normal moving temps are, because the Tstat will begin to close, and the fans will never reach thei turn off setpoint, and thus, never shut off.
So with the stock Tstat, you can probably set the fans to cool down to maybe 195° at best. (I prefer to have them turn off at least 10° higher than normal moving temps, but this can be pushed.
So, now with a tune, and perhaps some changes to the timing, lets say you want to keep it even cooler to prevent any knock retard from occurring, and run the fans down even more. This is where the lower temp Tstat is needed.
I have a 160 stat in mine. With it, I find my normal moving temps run about 176-178°...kinda tells me it's not really a 160 stat, but that's ok, I really don't desire it to run at 160 anyways.
I have my fans turning on at 195°, and turning off at 185°. My total window of operating temp is now 176-195°....right where I want it to be at all times. That would not be possible with the stock thermostat.
Hope that makes sense.
