It might be contended that the intrusion of the Stage Manager has the continuous effect of reminding the audience that they are not watching reality but are watching a play. This effect is also enhanced by the fact that there are virtually no props or backdrops. If anything has to be moved it is not done behind a closed curtain. Stagehands simply walk in and do whatever is needed. In the cemetery scene the dead people are not lying down but are all sitting straight up on wooden chairs--and yet this is the most moving scene in the play. We are especially moved by the presence of young Emily, who had such optimistic hopes and dreams and plans but died in childbirth. She doesn't seem to belong among all these old people who have lived their lives.
Thornton Wilder may have felt that the Stage Manager, or some such narrator, was necessary because of the experimental nature of his play. The audience could have easily become confused about time and place, since both keep changing. At the opening of Act II, for example, the Stage Manager says:
Three years have gone by.
Yes, the sun's come up over a thousand times.
Some babies that weren't even born before have begun talking regular sentences already; and a number of people who thought they were right young and spry have noticed that they can't bound up a flight of stairs like they used to, without their heart fluttering a little.
The Stage Manager provides most of the exposition throughout the play and thereby relieves the author from the necessity of conveying expository information through the dialogue of the characters. It may seem unnatural for the play to have an intrusive narrator, but at the same time it makes the dialogue and interactions of the characters more natural and realistic because they are not doing a lot of unnatural explaining to one another. Furthermore, there does not have to be a "plot," as such. We know we are just getting glimpses of small-town life which mirrors human life all over the world.
The Stage Manager is really the author, Thornton Wilder, in disguise. The Stage Manager tells the audience what the author would like to tell them himself. The play is not only tremendously moving but also a remarkable example of dramatic craftsmanship. It set an example for many other dramatists to follow. In Death of a Salesman, for example, Arthur Miller uses an impressionistic stage setting with minimalistic props.