To limit the probability that the failure of less robust traps will not be detected and corrected, which could lead to the leakage of sewage, which could lead to unsanitary conditions, which could lead to harm to persons.
To limit the probability that a lack of accessibility will lead to tubular metal or plastic traps not being cleaned, which could lead to the accumulation of solids in the trap, which could lead to waste water backing up, overflowing and flooding, which could lead to unsanitary conditions, which could lead to harm to persons.