It lets you win Special Alliance Medals that give attack and defence. You also win Cash Bonus rewards based on the performance of your Alliance. During some special tournaments, you can also win Gold.
At the end of the Tournament, Medals are awarded based on the ranking of the Alliance in the tournament.
Also, the personal performance of each Alliance member can be seen in the Alliance page. Based on the overall on you overall ranking in tournament special units are awarded to the top 20 players.
A player's personal best ranking is shown on his/her profile.