Ventilation is used in a house or building in order for the people inside those structures to get fresh air from the outdoors without going outdoors. Air conditioning units and heat pumps are constructed and attached to houses and buildings. Then the air is forced through vents into different areas of the house. Usually a vent covers the large gaping hole that leads into the room.
This is done so particles are brought into the room. It is filtered out. Ventilation is also used for certain appliances. A dryer must be ventilated so that the extra heat that it produces is forced away from the house. Even though people can open a window to let in fresh air, a ventilation system allows fresh air to come in without the bugs.