Primarily for the crew to see outside. The adjusting to light levels is often quoted, but after being dazzled by bright light it takes more than ten minutes for your vision to return to normal. So, it's for safety reasons. Mainly so the FAs can see outside and for those further away, so the pax can alert the crew to anything abnormal.
Quite a few case where pax have reported seeing something 'weird' to one of the crew.
For the crew, we can see quite a bit and it helps us to orientate ourselves as to where we are/how high we are. It'll also be one of the places we look out of in an evacuation before opening the door (viewing ports are on the door but if a window is immediately next to it, can provide a clearer view depending on the aircraft type)