Why airlines make you put the windows blinds up for take-off/landing
Why are airline passengers asked to lift up window shades during takeoff and landing?

I will answer this with a very long answer since aviation safety is my day to day job.

The window shade opening is a part of a long process to prepare the cabin for sudden (unexpected) emergencies. Why is that? Well, cabin crew have only 90 seconds to evacuate all passengers in case of emergencies. The process of evacuation itself should not take more than 90 seconds regardless of the size of the aircraft or the number of passengers. So to make the evacuation possible in 90 seconds cabin crew and other airline staff will prepare every flight in advance to make this possible. One example is the emergency exits, only healthy adults are allowed to occupy these seats, they are also required to read the safety card for emergency exits to make them prepared in case of emergencies to help the cabin crew. They are called ABPs (Able-Bodied-Persons). This is just one example.

Anyway, window shades opening is a small part of a long preparation process. Passengers are asked to open window shades before take offs and landings because these are considered to be critical times in aviation. Many things can go wrong during these times (most accidents do happen during take offs or landings), so people are asked to open window shades along with other things (seats in an upright position, people seated and buckled etc.) to make everything prepared as if the plane were in an emergency. In case something goes wrong (God forbid) things will be ready in advance so cabin crew can easily evacuate people in the short period of 90 seconds. 

Specific reasons behind the window shade opening include:


Passengers are curious, hence they are perfect extra eyes to see if something goes wrong out there. Usually, passengers report stuff right away.
In case of sudden emergencies, every second counts. Therefore if shades are open crew can easily see outside conditions to help them in planning the evacuation (which doors to use for evacuation etc.).
In case of emergency (which is more likely during take offs and landings) people should be prepared just in case. So during the daytime, opening window shades and putting cabin lights to full makes the eyes used to sunlight so if something goes wrong and passengers need to be evacuated there will not be a sudden change in light contrast which might lead to temporary blurred vision. The same thing at night flights, window shades are open and cabin lights are dimmed. 
It helps ground emergency personnel outside to see the inside of the cabin. 


These rules might be different from one airline to another, but in general, almost all airlines require their crews to make sure of the window shades during take offs and landings. They also add it in the preparation announcements prior to take offs and landings.
    