To limit the probability that an obstruction or deposit of debris in water supply pipes will lead to inadequate water flow, which could lead to negative effects on the operation of fixtures, which could lead to unsanitary conditions, which could lead to harm to persons.
To limit the probability that obstruction or deposit of debris in drainage pipes will lead to obstruction of flow, which could lead to blockage, which could lead to unsanitary conditions, which could lead to harm to persons.