chasv77answered grow question 4 years ago IMO, you really need some kind of circulation at all times.
As stated in the other answers, temps and humidity control are influenced a lot by your circulation.
But, I think you need to expel that air (oxygen), and bring in new (o2,co2) air for the plants to conduct their business.
For example, I have a grow cabinet that has a 2" input fan, and a 4" output fan through a filter. Those fans run ALL the time, while my separate 4" fan inside the cabinet, provides additional air, when the lights are on.