It is mandatory to keep window shades open while take-off and landing. These instructions are a part of the process to prepare the cabin for emergencies. In the case of an emergency, the cabin crew has only 90 seconds to evacuate all passengers irrespective of the size of the aircraft or the number of passengers.
Open shades help the crew to see outside conditions to help them in planning the evacuation. Open shades can also provide help through passengers because as some curious passengers keep looking outside, they are perfect extra eyes to see if something goes wrong out there to alert the crew to a problem in the flight.
It also helps ground emergency personnel outside to see inside the cabin. During daytime, it is advised to open the window shades and cabin lights are put to full to make people’s eyes used to sunlight so that in the case of an emergency evacuation, a sudden change in light contrast doesn’t lead to temporary blurred vision. For the same reason, in night flights, window shades are open, and cabin lights are dimmed.