To limit the probability that a hose nozzle valve will leak or fail, 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 the use of the hose will lead to static electricity build-up or sparks, which could lead to the ignition of vapour from the liquid transfer and dispensing operation, which could lead to harm to persons.