The Observer was there to, well, I guess just to “observe”! It didn’t appear like he was rooting for any particular team in the Giants/Eagles playoff game.
I just thought it was great for FOX to plant this little promotional nugget into the game. No credits were posted on the screen identifying The Observer. The announcers didn’t make any mention of it at all. There was just that slow camera pan, pointing him out silently, but clearly, to us who are always watching Fringe, observing The Observer.