The Stage Manager opens the play, and so he is the first character we meet. His entrance is memorable, as he recognizes that this is a play, and he begins to set the scene for us. Audiences might be expecting the action to just start, but instead they are given a narrator who recognizes them as the audience.
In the Stage Manager's opening monologue, he tells us all about the town: where the churches are, who the people are, what their daily life is like, etc. He also has omniscient information, such as when and how other characters die. Wilder makes the Stage Manager into a trustworthy character because he establishes that the Stage Manager has all of the information. The Stage Manager demonstrates his knowledge right from the beginning of the play.
The Stage Manager is our guide throughout the play, so we become very familiar with his presence. He answers questions for us and points out important details we should pay attention to. He also steps into the role of various townspeople, such as the minister, making him more human. The Stage Manager is Wilder's voice. He shows us the important themes.