Consider this, if you have a mobility impairment, how do you leave a building upper floor?
You may well have entered the building aided by lifts, escalators or platform lifts, however they are unlikely to be available during an emergency, therefore escape will be by means of a protected stairwell.
It is intended that persons requiring assistance will congregate at a place of relative safety; such as a protected stairwell. These areas are referred to as a refuge area. From here assistance can be summoned by the use of Emergency Voice Communication System or EVCS. A safe and orderly evacuation from the refuge area can then be carried out by the building management. Utilising specialist evacuation chairs etc by trained personnel.
These systems are called for in BS 9999: 2017 and Building Regulations Approved document B (Fire Safety) Volume 2: Buildings other than dwelling houses.
A refuge area is a relatively safe waiting area which should be provided at each protected stairway affording egress from each storey.