What is the safety reason that airplane windows have to be open during take off and landing?

Improved visibility might be helpful in case of an emergency. Specifically, the occupants must be able to see outside to assess if it is safe to open the emergency exits. This is especially important in emergencies that invovle water landings (if an exit is below the water line) or in cases of an external fire, where the act of opening an exit will expose the occupants to the fire.