As you probably know, since the 50s that most films are shot for a 1.85 or even narrower (2.2, 2.35, 2.85, etc) aspect ratio, meaning an image much wider than the 4x3 (1.33) aspect ratio of ordinary television sets. So while films on television should obviously be letterboxed (meaning the addition of black padding outside the film frame), stupid viewers everywhere (I’m sorry, there’s no other way to put it) demanded the ‘stolen’ area of their TV sets back, which gave rise to the practice of doing ‘pan and scan’ reedits of the films, with the frames (and many times the actual editing) readjusted for 4x3 screens.
I absolutely hate watching pan-and-scan films. You keep all of your TV set’s pixels in use and instead it’s the actual film that’s being stolen (as if advertising breaks weren’t annoying enough). This video pretty much explains it all.