To limit the probability that building services [e.g. gas, electrical and heating installations] will be damaged during excavation, which could lead to the start or spread of fire, which could lead to harm to persons.
To limit the probability that open valves will lead to the accidental release of liquid or vapour [e.g. from gas or fuel lines], which could lead to the accumulation and subsequent ignition of vapour, which could lead to the start or spread of fire, which could lead to harm to persons.
To limit the probability that the type of service will not be readily identified, which could lead to the misuse of the services [e.g. the unwanted opening of a valve and release of liquid or vapour], which could lead to the start or spread of fire, which could lead to harm to persons.