They leave the fuel tank open to breathe to avoid dangerous build up of fuel fumes.
The picture shows a fuel filter used to ensure fumes can get out, but no containments can get in!
You will also often see a sign saying "No Fuel" hanging on a tether attached to the car, usually high up on the airbox. This verifies that someone has emptied the fuel tank when it came into the garage. It also reminds the team to put fuel in before trying to start up the engine.