A portable air conditioning unit requires a vent because it needs some way to remove heat from the room it is cooling. Without a vent, the unit simply discharges the heat back into the room, resulting in no net cooling and wasted energy.
An air conditioner cools by transferring heat from one place to another. It removes heat from the air that passes over its evaporation coils and discharges that heat from its condenser coils. The standard method of venting a portable air conditioner is using a vent kit that connects to a window, since it allows the excess heat to travel outside. If a portable air conditioner has no access to the outdoors, the heat can be vented into an air duct or drop ceiling, away from the contents of the room.