To limit the probability that the heating system will be an ignition source, which could lead to the ignition of nearby vapour from the piping system, which could lead to a fire or explosion, which could lead to harm to persons.
To limit the probability that liquid in the piping system will be overheated, which could lead to a failure of the piping system to contain the liquid, which could lead to the escape of liquid, which could lead to the accumulation and subsequent ignition of vapour from a nearby ignition source, which could lead to harm to persons.