Symmetry is not always necessary, but can often help even the playing field. It gives both teams the exact same opportunities and lets the matches be determined by skill. This balance is what makes a game not only fair, but fun. As well, fairness is what makes a game well designed. If a game gives poorly thought out advantages to one team, that game is not fun to play and will likely not be played much. The same goes in singleplayer, too.