A problem that can be very serious for a homeowner is when they experience backflow. Backflow is caused by a sudden drop in water pressure. This could happen due to a burst main water pipe, a fire hydrant being used by the fire department, or because of some natural disaster. Whatever the cause, the drop in pressure can lead to sewage being pulled back up the pipe, into a home. You can just imagine what havoc that could cause in your home, and this is why a device called an atmospheric vacuum breaker is used. These pipes are built over the ground, and their purpose is to pull air into the system and thus preventing backflow. It basically is a simple valve that lets water flow during normal operation, but if there are possible backflow conditions, it will instead let air in and break the vacuum.
While it is a simple construction, it is a vital part of a functioning water supply system.