To limit the probability that liquid or vapour will escape through the opening, which could lead to the ignition of vapour from a nearby ignition source, which could lead to harm to persons.
To limit the probability of tampering or entry of foreign objects into the tank, which could lead to the ignition of vapour from a nearby ignition source, which could lead to harm to persons.
To limit the probability of entry of precipitation into the tank, which could lead to the escape of liquid, which could lead to the ignition of vapour from a nearby ignition source, which could lead to harm to persons.