To limit the probability that inadequate frost protection will lead to the contents of pipes freezing, which could lead to pipes rupturing, which could lead to leakage, which could lead to unsanitary conditions, which could lead to harm to persons.
To limit the probability that inadequate frost protection will lead to movement of the pipe bedding and backfill due to frost heaving, which could lead to misalignment and damage to the piping and joints, which could lead to leakage, which could lead to unsanitary conditions, which could lead to harm to persons.