To limit the probability that persons using stairs will have difficulty in securely placing their feet on the steps, which could lead to difficulties in movement in an emergency situation, which could lead to delays in the evacuation or movement of persons to a safe place, which could lead to harm to persons.
To limit the probability that persons using stairs will have difficulty in securely placing their feet on the steps, which could lead to persons tripping or falling, which could lead to harm to persons.