What is the role of the Stage Manager?

With most plays the audience has to obtain all its factual information from what the characters say and do. In Thornton Wilder's experimental play Our Town, the Stage Manager provides most of the explication. For example:
This play is called "Our Town." It was written by Thornton Wilder; produced and directed by A. . . . (or: produced by A . . .; directed by B. . . .) In it you will see Miss C. . . .; Miss D. . . .; Miss E. . . .; and Mr. F. . . .; Mr. G. . . .; Mr. H. . . .; and many others. The name of the town is Grover's Corners, New Hampshire--just across the Massachusetts line: latitude 42 degrees 40 minutes; longitude 70 degrees 37 minutes. The First Act shows a day in our town. The day is May 7, 1901. The time is just before dawn.
The purpose of providing all this mostly superfluous information is evidently to acquaint the audience from the outset with the unusual method of directly conveying explication through a narrator and of making it plain that this is a play, rather than attempting to hide that fact by creating the illusion of reality. Specifying the precise location of Grover's Corners as

latitude 42 degrees 40 minutes; longitude 70 degrees 37 minutes

is the author's way of showing the audience that the Stage Manager knows everything and will be telling them everything throughout the play. The narrator is a character in the play and is called the Stage Manager in order to remind the audience that this is a play. The audience is spared the work of guessing and piecing information together. The author is relieved of the problem of conveying explication mainly through dialogue, which can often sound unnatural and undramatic.
After all, the audience knows they are watching a play. Why try to fool them? This is a distinctly modern approach and has been quite influential in dramatic works ever since Our Town was first produced in 1938.