Dangerous super-heated gases need to be ventilated to allow firefighters to safely and quickly rescue trapped occupants and extinguish the fire. By venting the window (horizontal ventilation) of a room that is on fire, it actually helps to contain the fire within that room of origin. Otherwise heated gases spread throughout the inside of the house. Breaking the window really prevents a great deal more damage than it appears to cause. Replacing broken glass is much less expensive than repairing structural damage from the fire.