A check valve is used to prevent water from siphoning out of your aquarium. If water siphons out, it will usually damage the air pump and lead to water on your floor. In extreme cases, this can lead to fire if electronics are sitting in water.
A Must-Have for Every Air Pump
A simple check valve is all that stands between you and a floor full of water. If your air pump is below the top of the tank, you absolutely must add a check valve between the air pump and the air-driven device (e.g., sponge filter or aquarium decor). If the air pump ever loses power or gets turned off, the check valve prevents water from traveling down the airline tubing due to gravity.
Don't Forget About the Brine Shrimp
A common "air-driven device" that people often forget about is their brine shrimp hatchery. We have personally lost power in the middle of the night and experienced the pain of cleaning up fishy, rotten brine shrimp all over our counters. So don't forget to add that check valve!