These rollover vent valves are designed to shut off flow through the vent valves in the event of a vehicle rollover. This uses a ball check valve to allow flow when the vehicle is right-side-up, and closes with gravity to prevent fuel from escaping when things don't go as planned. Required by many sanctioning bodies, particularly if you're not using the factory fuel tank.
This prevents fuel from draining out from your vent line, which can lead to a fire.
