To limit the probability of vapour accumulation in the contained space, 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 delays or ineffectiveness in firefighting efforts, which could lead to the spread of fire beyond the point of fire origin, which could lead to harm to persons.
To limit the probability that a tank defect or leak from the tank will go unnoticed, which could lead to the escape and spread of liquid, which could lead to the ignition of vapour from a nearby ignition source, which could lead to harm to persons.