It keeps out alien mind reading radio waves.
Why put aluminum foil on windows?
I use it during the summer, on my southern windows, to keep the house cool ... Since I don't get any "cross breeze" by opening windows, and I don't have AC. I find it keeps the inside about 10 degrees cooler than outside.
I think most people use it to reflect sunlight and heat away from the interior of thier home. Helps to reduce extreme heat.