To limit the probability that a fire involving the liquid will grow and spread, which could lead to the spread of fire to other parts of the building, which could lead to damage to the building.
To limit the probability that the piping will be exposed to a fire originating in its covering material, which could lead to the failure of the piping, 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 damage to the building.