What is little plastic cap on an automatic air vent for?

I understand automatic air vents like the one used next to the boiler in my domestic heating system work by having a float operated valve that opens when air builds up inside the vent, but close when that air is expelled. Therefore, if I understand correctly, water should never come out of the hole at the top, only air. What is the purpose then of the little plastic cap that is screwed on to the top of the valve, and should it be tight or loose? Any help much appreciated - these things have baffled me for years!