The air flight attendance once told me that we're not allowed to put down the shade when taking off and landing is to allow everyone to have a view on that outside to ensure nothing went wrong with the plane such as smoke coming out from the wing and etc. If a snoke were to be seen, at least the flight attendance can immediately contact the pilot to warn them, then to wait for the whole wing to explode. That will be too late. That's what she told me.