In the event of a fire, minimizing the spread of smoke and heat within the building is essential. It is usually the smoke and heat produced by a fire which limits the ability of people to evacuate and which causes the most deaths in a fire.
In most modern buildings, air handling systems separate from the building's main HVAC systems can be activated to contain the spread of smoke.
Stairwells in newer high rise buildings are pressurized to maintain an atmosphere that is both safe for exiting and an aid in fire fighting procedures. In older buildings, the HVAC systems can be manually controlled either to shut down in order to prevent the spread of smoke through the systems themselves or activated in particular portions to remove smoke.
Imagine how much air can be moved by a system of this size.