why is it that lights in the aeroplane have to be switched off when alighting

just a precaution.
try ask Adam and Jammie (mybhnusters) about this haha

We call it landing. To stop any glare onto the runway.

Standard Operating Procedure.

Landing being one of the more dangerous parts of a flight, dimming the cabin lights will allow passenger is eyes to adapt to darkness in case there is a need for an emergency exit from the plane in darkness or in smoke (say, after a plane skidded off the runway).