To limit the probability that doorways will be of insufficient height, which could lead to persons contacting or colliding with the top of the door frame, which could lead to harm to persons.
To limit the probability that doorways in exits will have insufficient headroom clearance to permit efficient egress 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 supersede the requirements of Sentence 9.9.3.4.(1), which would otherwise require a greater headroom clearance in exits, and permit a reduction, on the basis that the reduction is limited and should not negatively affect exit usage.