To limit the probability that excessive pressure will build up in the receiving tank, which could lead to failure of 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.
To limit the probability that excessive back pressure will build up in the receiving tank, which could lead to overflow from 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.