Your tank has to breathe. In order to breathe, your tank must have a vent somewhere that will relieve both vacuum and pressure. Gasoline expands in volume as it warms up and shrinks in volume as it cools down. The fuel level of your tank changes throughout the day, even if you are not driving it.
You cannot put fuel into your tank, unless you can get the air out...and you cannot withdraw fuel from your tank unless you can let air in.
Up through the 1960's, most vehicles used vented gas caps. This is simply a gas cap with a hole in it. Unfortunately, this hole would allow the fuel to splash out when accelerating or turning a corner.