To limit the probability that a person will step through the door down onto a step, which could lead to the person falling, which could lead to harm to the person.
To limit the probability that a person will step through the door down onto a step in an emergency situation, which could lead to the person falling or obstructing other persons using the exit, which could lead to delays in the evacuation or movement of persons to a safe place, which could lead to harm to persons.