In my option it's temperature difference.
I personally don't ever need an intake on any tent I've set up 8x4 square feet or below.
But someone in a hotter climate might.
Or someone running higher wattage, like 2x 1000w hps compared to 2x 600w LED.
If my desired temp is 79f and my ambient room temp is 69f then I have a reasonable amount of room for that increase and it won't require huge airflow to do it.
But if your ambient room temp is 75f and you desire the same 79f in the tent then you need a considerable amount more and you may get to the point where intake is needed or your tent would be under considerable pressure.
At the end of the day the wattage used and ambient temp dictate the requirements for extraction.( In an active air grow space, As opposed to a sealed room)
An open duct or tent flap allows a high amount of air passively.
Obviously other than that an intake allows all flaps (light leaks) to be closed , which would matter to some people, so that's another reason to have an intake.
But it has to be reasonably balanced because if the only way air can get in is through an intake fan then that can be a limiting factor , because it will limit how much air can be sucked out of the extraction. In that case the extractor is working too hard.
I'm sure a hvac guy would know more about the specific rules , but that's the general idea