It depends on the situation and what is being removed/ ventilated!
positive pressure ventilation needs an adequate escape route for smoke or some light gasses,
but a heave gas or mist of tear gas needs the use of positive pressure and exhaust (negative pressure)
these are required to get a large volume of air moving to disperse the gasses and or mist.
most ventilation needs that firefighters encounter is smoke removal! smoke tends to be heated and will readily rise! therefore attic / roof/ gable or windows offer sufficient escape,
heavy gasses such as co2 or tear gas however do not rise readily and ground floor exhaust and positive pressure are required and will disperse them quickly.