First that doesn't incorporate the main body of sport fans whom watch from home. Second it doesn't take into account that there is going to be fans for the other team present as well. So I find the 'moral boost' to be balanced out and even if it is not... these are professionals. If they are not going out in the field with the mentality that they have already won then no amount of cheering will help them win against a team that has already decided victory is theirs before the game even begins. Sun Tzu.
Fans who watch at home can't do anything, but the mentality is the same as to watching movies. You can still get a rush from action movies, and you can feel emotions from a drama. Sports can make you get very excited, and make you feel emotions. This can be very hard to explain to those who aren't interested in sports.
I never really had this effect happen, makes it harder for me to understand, but the chemical reaction to watching a sport is why I am asking why.
The chemical response is the effect, not the cause, but why does it do that? I guess we would need a whole bunch of neuro-scientists and psychologists to just explain what this chemical reaction does. As to why we have such responses to events we are not taking part in? Interesting to ponder on though.
I think it can be the cause, as many wish to experience the rush. I'll give an example to this:
You masturbate for the feeling at the end, not for the path getting there. (Quick example, don't read too much into it >.<)
Oh the (misused version of) irony that, for many, it is the lack of sports that put them into this position.
Yes, but most cannot compete at that high a level regardless of their current physical condition. Watching someone else do something that you can never do is a reason some watch.