Without the external lights people would have difficulty finding their way.
Previous editions of BS 5266-1 allowed the use of street lighting to illuminate the external areas. With local councils turning off street lights at certain times to save money and energy.
It’s not always possible to rely on the street lighting. The current edition of BS 5266-1 says that if street lighting was relied on it needs to be reassessed to make sure that they will still be illuminated at all times the premises are in use.