To limit the probability that a mix of rectangular
and tapered treads in a flight will disrupt the gait of users, or
force them to switch to the other side of the stair, which could lead
to:
persons misstepping and tripping, or
persons misstepping and tripping in an emergency, 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.