This is because conventional plumbing systems include pipes above the fixtures which go through the roof where make-up air is available.
Unfortunately, the pipes make holes in your ceiling and roof, and the holes are difficult to seal completely. In winter, air within the home is typically far warmer than outdoor air. The air a heating system makes warm and comfortable tends to leak out any cracks and gaps around these pipes. Thinking about the home as a hot air balloon in winter, the pipes are gaps where warm air leaks from the balloon.
In addition, air in waste water pipes underground is cooler than the air in a home, and as the home warms the pipes and the pipes warm the air within them, it rises and travels out through the roof—bringing more cold air into the pipes and cooling your home.
Thinking about the home again as a hot air balloon in the midst of much colder outdoor air in winter, the pipes are holes through which costly warm air is constantly escaping. The pipes in the conventional plumbing system are a virtual conveyor belt continuously bringing cool air into your home that cools it in winter while transporting air that you have paid to warm out through your roof. That means continual heat loss all winter!
An alternative source of the small amount of make-up air needed for home plumbing to work properly is the air within the home. However, you do not want the air in the pipes to flow into your home---only the other direction.
AAVs allow small amounts of air from inside the home to enter the pipes as water flows down them and out of the home to the city sewer system or home’s septic system. At the same time, AAVs automatically close when air is not drawn into them in order to prevent smelly and dangerous sewer gas from coming into the home.