In Act III scene i, which charts the fight between Tybalt and Mercutio and Mercutio's unfortunate demise, it is clear that from the very first entrance of the Capulets, Benvolio plays the role of peacemaker. His main speech comes before Romeo enters and Mercutio and Tybalt have begun to insult each other:
We talk here in the public haunt of men.
Either withdraw unto some private place,
Or reason coldly of your grievances,
Or else depart. Here all eyes gaze on us.
Note how his speech attempts to diffuse the explosive situation by reminding both parties of the public gaze upon them all. Given this audience, it makes sense to withdraw to "some private place" or to speak rationally and reasonably to each other about the "grievances" that exist between them.
Thus Benvolio plays the role of rational observer, trying to stop the potential explosion from breaking out and limiting the damage.