[aeternum]

They used to ask you to open the window shades for takeoff and landing, idea was to actually be aware of what is going on outside incase an evac is necessary.

Now they ask everyone only to close the window shades after landing to better control the temp, the APU a/c is often no match for direct sunlight. Then most people just leave them closed.

[RubberbandSoul]

I've heard the opposite. Keeping the shades open is so that the rescue crews can look into the plane in case of a crash/accident.