from fans at the stadium, people who are driving on the road, or basically fans doing any type of activity that you can't do sitting in front of a tv (exercise, work, etc). I think most fans (whether you are at the stadium or not) like tuning into the pre and post game shows as well. Depending on who is doing the call, I will sometimes mute the TV call and turn on Wrubell as well.
I personally think the radio play-by-play job is way better than a BYUtv analyst.