to keep it from fogging. actually the window and outside walls are the coldest so that is where you want the heat so you don't get the wall sweating/moisture buildup/mold growing in the corners or on the windows.
"Cut it twice and it is still too short".
Ever sat next to a window during the Winter or Summer? My place, the heat vents are along the inside wall, due to back in the 1930's, 1940's, they did not know about the effect of how heating works with personal comfort. I ended up placing 3m window film on my window behind our couch, because you could just feel the cold air radiating off of the glass, and these are double-pane with Storms.
Actually Yuri, if you are getting moisture on the window, that would be due to the house being too tight. The fogging can be a mixture of partial air loss, and too much humidity along with the temp being just right. Even with our storms closed, and the 3m window film, you can see the storm windows fogging, due to the trapped air between the house windows, and the storms. The trapped air temp is just right, but the windows are not fogging, due to to there is no air loss and the humidity in the house is 58, and temp is 68.