What is the function of the Chorus in tragedy?

The Chorus is one of the most interesting and unique aspects of Greek tragedy. It is, on the whole, made up of citizens of the city where the play is taking place, and they voice their concerns and thoughts regularly. In terms of their function in the play, they act as commentators on the action, whilst never really being able to influence the plot or the characters. They exist to pass comment, and they do this is two main ways. Firstly, they sing the stasima, a kind of interlude between sections of the plays. These might be directly related to the action that has preceded the stasimon, it might be about what is coming next, or it could be about something seemingly unconnected to the play, subversively passing comment.
Their function is to pass comment, but what is more important is why that is their function. The Chorus is, really, the moral guide of the audience through the play. The Chorus would have been in the orchestra, which was directly between the main stage and the audience. They are both a physical and metaphorical bridge between the audience and the action, and so they can guide the audience, making them see certain characters in particular ways.