There was a time that TV stations did not continuously broadcast 24 hours a day. After the late news and the late shows, the station would sign off, as if to give it (and the viewer) a much needed break to rest and recharge for the next day of television viewing.
As a child, I got a real sense of achievement if I could stay awake until the conclusion of the broadcast day. It was if I had outlasted TV in some way. I would watch with a sense of pride the images of fighter jets superimposed over waving American flags as The Star Spangled Banner played. Then the song would end, the images would fade and the TV picture would abruptly cut to static. It would be then, bathed in the flickering glow of static, that I would realize exactly how late it was, how dark it was, and how completely alone I was. Suddenly, sneaking into the farthest room from my parents bedroom to watch TV didn't seem like such a great idea.
There is something inherently scary about static. Maybe it's the way static moves and sounds like insects that triggers some deeply buried cultural memory. Maybe static is scary because nothing good is associated with it. If you see or hear static you instantly know that something is wrong. The most famous, and in my opinion the best, use of static to set a scene is in POLTERGEIST (1982). When you see Carol Anne crawling towards the TV full of static you just know that no good can come from it.