To limit the probability that excessive variance in the
configuration of a stair will excessively disrupt the gait of users, which
could lead to:
persons misstepping and tripping, or
persons misstepping and tripping in an emergency -- when traffic
flow is expected to be limited owing to the presence of exit stairs as an
alternate option or to the limited number of occupants in dwelling units --
which could lead to delays in the evacuation or movement of persons to a safe
place.
This is to limit the probability of harm to
persons.