Benvolio is who I call the "peacemaker" of the group. In that very first scene where the fighting occurs, he's the one who jumps in shouting "Part, fools! Put up your swords; you know not what you do." In other words, he's trying to separate the fighters, telling them to put down their swords, and pointing out that it's their tempers making the decisions at this point. He's the one who sees that the reason for the fighting is, to be blunt, idiotic. He realizes that everything is getting out of hand for no good reason.
After the initial fight, it's Benvolio who is left to explain the course of events to the Prince. Tybalt's not doing it, and neither are Sampson, Gregory, Abraham, or Balthasar. He gives a pretty succinct description of the fight, how it got started, his role in it, and everything else.
One other, albeit small, example is at the end of the act during Scene 5 when it's Benvolio who attempts to pull Romeo out of the party, letting him know that "the sport is at the best," or we're done -- party's over, we have to leave.