To limit the probability that certain areas in buildings will have inadequate illumination when there is a loss of normal power, which could lead to safety hazards [bumping, tripping, falling, etc.], which could lead to harm to persons.
To limit the probability that certain areas in buildings will have inadequate illumination when there is a loss of normal power 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.