Another may be good at coordinating everyone's scores and keeping all the members from being paranoid about each other. That is very important because distrust can destroy an alliance in a minute. Ideally any alliance should work towards a result as close together as possible. It is very easy to take advantage of an alliance and push yourself into a win at the expense of the other members. But you might wind up playing with the same player in another game.
People are willing to forgive their enemies.
When in direct confrontation you fight it out and the best man wins. Your bitterest opponent in one game may be your staunchest ally in another as long as you know he keeps his promises. If an ally stabs you in the back that is a very different story. If you meet him in another game you have a guaranteed enemy who will do his best to wipe you out as fast as possible. But it CAN be used for valid reasons.