Sometimes I want to watch it, sometimes I like to have it in the background.
If there is a baseball game on, for example, you don't really need to focus on the entire thing, even the commercials, for the duration of the game, you can put it in a window while you do something else and then return your focus to it.
Personally I think you are lying that if something is on the tv, you do nothing except for watching that thing on the tv screen. Even checking the TV guide counts as doing something else, and I doubt that you shush anyone else in the room who might ask you something. Do you also stare at the screen when there are commercials? Remember if you do anything except for staring at the screen, you are no longer paying attention to something you have the desire to watch.
To be fair, it's possible that you are so bad at multitasking that you have to focus at one single task at a time. In whic case I apologize about saying you lied in the previous paragraph.