Take Word, or Visual Studio or Photoshop as examples. When they are in the background they aren't drawing to the screen, they aren't doing stuff in the background
You must have a different copy of Word to me. I can sit here and watch Word happily animating a little icon in its status bar to tell me the proofing tools are busy checking a blank document, despite me doing nothing at all. I have no idea what the proofing tools think they're doing, but I know for a fact the CPU is involved in constantly redrawing that little animation, keeping it hot.