I intentionally phrased the question title a bit provocatively. What I mean is this: Aperture priority mode (Av) means that I select an aperture, and the camera selects the corresponding shutter speed. Shutter priority mode (Tv) means that I select a shutter speed, and the camera selects the corresponding aperture. Right?
So, for a given static scene, by me selecting any given aperture in Av, the CPU sets a corresponding shutter speed. And by me selecting any given shutter speed in Tv, the CPU sets a corresponding aperture.
Can someone please explain to me how the two things are any different?
Indeed, when I try it out (I have two different EOS Cameras, both are 35mm film SLRs: EOS Rebel Ti and EOS 620), I get either exactly the same, or very similar, shutter speed / aperture value combinations, regardless of whether I'm in Av or Tv (for the same static scene on a tripod, with static light). I can even set Av and "pretend" I'm in Tv, and the other way 'round.
So why are there two different modes or what's the difference between them? And isn't Program Shift, too, essentially the same for all practical intents and purposes?