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.
Best practice is to place the check valve near the top of your aquarium. This stops the water at the rim instead of down near the pump where over time the pressure could cause a leak. Make sure your airline tubing is cut nicely and free of any tears. Check to make sure the tubing hasn't dried out over time causing the connection to be loose as this will leak in the event of a power outage.
I would definitely recommend adding these little guys to your tank. They are very playful and entertaining. My three were very healthy and quickly adjusted to my tank.
